Click below for  a higher Res. Picture

FIRING ORDER - Maintenance Manual X-7525 Figure 50 on page 6Y-50 is
incorrect! The # 3 and #6 posts in the distributor are reversed. Note: all
other figures in X-7525, X-7625, and X-7725 are correct.  ROB

To search for a topic on this page type Ctrl+F and the key word.

 I have added some pictures showing the props in more detail.  I purchased the part from McMaster-Carr.

P/N 9416K192 are the gas spring 30lb force
P/N 9416K86 are the end sockets
P/N 9512K73 are ths ball mounts
I used aluminum angle to mount.
Measuring from the rear it is 26" to the ball on the hatch and 11 1/2 on the floor
 I don't use a hinge, the hatch fits into a slot in the front. If I need to remove the hatch I just pop the gas springs off the ball and lift out.
 Hope this helps
 It eliminates the embarrassment of explaning why there is a knot on the back of your head and the inprint of the air cleaner wing nut on your forehead
Jim Wagner
I put some pictures up on my website that I took at the rally in Iowa.  I  hope you don't mind. I'm going to place an order with McMaster-Carr today
for the parts.
Richard Waters
Supplementing the September 2007 issue of GMC Motorhome News, Sam Carson of Indian Harbour Beach, Florida, has advised that new  American Petroleum Institute (API) SM Service Category oils contain a significantly lower amount of zinc dialkyl dithio phosphate (ZDDP) than former API SL, SJ and earler API Service Category oils.  The ZDDP component is an important additive for the lubrication of flat lifters and cams in GMC Motorhome engines.

Carson is aware of at least three GMC Motorhome lifter and cam failures after owners changed to SM oils, which are the oils most retail outlets currently stock.  Our search of the media indicates similar failures in other engines manufactured prior to the 1990s when roller lifters were installed in almost all new large-block engines.

The ZDDP  additive has been significantly reduced because the Environmental Protection Agency (EPA) claims it can poison modern-day catalytic converters.  The petroleum industry is conducting tests to
either prove or disprove this claim.

We have long recommended Mobil 1 for GMC Motorhome engines, but many owners do not want to pay the price of synthetic oil.  Synthetic oils have not been reformulated and meet either API CF or CJ-4 diesel engine Service Category specifications along with API SL or SM Service Category specifications.  These oils maintain old levels of ZDDP, and will adequately lubricate the lifters and cams in GMC Motorhome engines.  Other non-synthetic diesel engine oils that meet API CF or CJ-4 Service Category specifications also have adequate levels of ZDDP.  Chevron's Delo is an example.

Since all GMC Motorhome engines were manufactured with flat hydraulic lifters, API SM Service Category oil "should not" be used when changing oil unless it also meets API CF or CJ-4 diesel engine
Service Category specifications.  To be on the safe side, use Mobil 1 15-50.  It is expensive, but often can be purchased on sale at discount auto parts stores and at Wal Mart.
Cinnabar Engineering, Inc.

GMC Motorhome News is available at:


Your engine problem sounds like the failure I experienced last fall returning from Nashville.One cylinder measured 10psi compression and a oily spark plug.One third of the exhaust valve burned away in 10,000 miles since complete engine rebuild.

Only one head was removed and another head installed in under 8 hours Repair was done in my garage.Required one intake manifold gasket replacement on the left head Lawrence

Examined the plugs, good Color. Then a compression check showed 150 for all the cylinders except 6 and 8. They were at 30 pounds.

Removed intake including carb disconnecting as few vacuum lines as possible. Then disconnected exhaust pipe so that themanifold came out with the head. Much easier to remove manifold bolts on thebench.When we go back together I will install the manifold on the pipe loosely.Then mount the head and torque bolts to specifications. Then using the twoend bolts I will attach the manifold to the head loosely. Then insertexhaust manifold gasket between head and manifold, install the rest of themanifold bolts and torque to specifications. And so on and so on. If I wasonly removing the exhaust manifolds I would remove the wheel well liner and
front wheel. Much easier............Terry in Tacoma
Here are the pictures


 Names have been removed to protect the Guilty

I just unhooked the line at both ends and capped the TBI line.  When I went for my smog I replaced the carb, installed the line and pushed the line over a cap on the carb.  I didn't want any extra HC present.  Passed with flying colors.  The canister is only there to catch evaporation.  It is not part of the tank venting system.

last spring I experienced the symptoms that you have and I disconnected the line from the separator valve, plugged the port on the carb and ran a line from the separator valve down along the frame to below the bogie wheels.  I do not have to have my vehicle inspected here in Santa Fe but if I should move I can easily reconnect it.

I have the Caspro Stainless Steel vents on the coach.  Yes they do work and work very well.  Anything that you can do to remove the heat from the engine compartment is good.  I can stand next to the coach and the air flow just pours out.  Along with Caspro, Jim Bounds sells a less expensive unit that works very well also.  Some people have also used the vents list on page 544 of the West Marine catalog.  Model numbers 375477 & 386559 have approximate the same dimensions as the Caspro unit but are half the price.  You can go to www.westmarine.com.-- J. R. Wright

I've been looking at several photos of coaches that have been "updated" for ideas. That includes photos of the inside and photos of the outside. I understand that a big concern is heat in the engine compartment. Based on my unrealistic research it appears that about 45% of you may or may not have added fender vents (how's that for a neutral political statement?)

Based on the percentage of you that have added vents it would appear that there are several varieties and styled implemented with the Ragusa not being quite as popular as I would have imagined. So, I set about in search of some options and came upon several at West Marine.

Now, the Silver Lady is in the shop undergoing a lung transplant so I can't go out and measure her cavity. I'm turning to the next best thing and seeking an opinion via this illustrious body (and you have to be to own a GMC). The question is, will this vent fit? It's takes a 3 5/8 x 12 in. cutout.

The URL is too long to put into a text document. You can, however, go to http://www.westmarine.com and follow the links for "Ventilation" / "Exterior Vents" and seeing what I'm talking about. It's on the right hand column. The caption says "SEAFIT VENTS | Stainless Louvered Vent - 13"L | Only $21.99 USD"

Looks descent. Good price. BTW, I did find the type like is on the Cadillac SUV but I don't want to pay $230 for a chrome version when this may do. Byron Songer

Back in 1990 I got some nice looking stainless steel vents and put them into the side.  I must say that when you stopped you could certainly feel a lot of hot air coming out of them.  However, I never knew if they did anything when driving down the road.

A few years back I installed some of Darren Paget's insulating blanket pads under my floor and engine box cover.  That really deadened the noise and the floor was much cooler.   However, I then found that I was starting to get vapor lock which I had never encountered in several previous years.

An examination of this new problem revealed that the fender liners have a "standoff" where the screw goes up into the floor above the liner.  If you examine this closely you will see that the liners have a "scoop" on the front which brings air up and over the liner and down to the back.  This is something like an airplane wing shape and the air going over the liner (think wing) is going a farther distance in the same amount of time as the air under the liner (think wing).  This creates a low pressure zone at the top which in the case of a wing gives lift.  I feel that in the case of the fender liner the lower pressure will pull the heat from the manifolds which are adjacent to the wheel liner.  When I blocked this passage with the insulation it no longer pulled air over the liners and air from the motor box so I started to experience overheating and vapor lock.

My solution was install some louvers in the wheel liners (house foundation block vents) and also to put some scoops under the bumper (house rectangle to 4" round duct adapters from Home Depot.  The ducts have some aluminum dryer hose that goes to the back top corners of the engine box to force cool air into the top of the box which in turn pushes out the hot air.

All I know is that it has worked extremely well.  I live in the hot southwest and often travel in 120 degree temperatures while pulling a toad and have never had any vapor lock since doing my modifications.

You can see this at:

They are not in the original order but there are not many pictures.Emery Stora

Here are Bert's wheel well holes

Yes I have the serpentine setup on my coach. (Now has been removed 6/29/05)

I had a few problems with the a/c compressor pulley. I was shipped a pulley that would rub against the body supporting post. This led to bearing failure and clutch failure while on the road. After I complained about it I was asked to use a shorter belt and given the part number. A new pulley was shipped, shorter than the previous one but once installed, no way to pull it out without damaging the groove ring, I shipped it back. I ended up machining a pulley and using the ring from the first pulley. I also had some alignment problems with the alternator/power steering units mainly due to a missing bracket. Emery helped out on this on the parking lot in Mt Hood on the GMCMI  rally Water pumps: I did a study on water pumps and the reverse
flow and had posted on photopoint. I found out that with the reverse flow I was only getting 2/3 efficiency. My solution to this was replacing the thermostat with 170. For those interested, I would not do it again. I would not go back either because its working fine and I spent too much time getting it to work. Manny

Jim Bounds
I tell ya guys, you know when you ever make a decision and get off of the proverbial fence--- there will always be someone on the other side that will not be happy with your move but hey--- crumbling cookies!  After talking to many about this both professionals, engineers and even people I respect an opinion of---- we are not going to reinstall the serpantine belt system on this motor.  I did not make this decision lightly and if I can bore you for a minute, I'd like to tell you why we are doing this.   (6/20/05)

The best spark plug wires are Dick Paterson's(http://www.paterson-gmc.com/wire.htm).  Spiral wound wire cores have minimal resistance compared to the graphite core OEM types.  Spiral winding "chokes" RF interference.  For the price he charges, you couldn't build your own from the "universal" kits in JEGS, etc.  My $.02 Patrick






Here's how to put it back in.  Removal is sorta the opposite. :-)

You will still need a gantry from which to suspend it, though it need not be
as tall as the manual shows -- just tall enough for the two winches to
operate beneath it.  The gantry should have two hoist trolleys and the hoist
must be strong enough to individually support the engine+transmission+final
drive -- certainly no less than 1-1/2 ton for safety.  Position the stronger
hoist at the forward mounting point, just forward of the CG (~3" fwd of the
forward carb mounting bolts).

You'll need to get the coach 30"+ off of the ground.

If it weren't for all the wires & hoses, it would be an easy job.  I spend
more time trying to find & correct the leaks than anything else. KenH

how not to do it ;>)

I first raised and supported the coach about 6" higher than normal ride height. Then I removed the following (text from Jim Wagner's post):

> I first removed bumper, grill, valance, inner fenders, battery tray, air
> pump, steering box, brake lines, gas lines and gas filler tube from
> tanks, emergency brake cable, misc. wires + normal wires and hoses
> needed to remove motor through floor.

Also unloaded the torsion bars, unbolted exhaust system from manifolds, discharged A/C system and disconnected the lines from the evaporator, disconnected speedometer cable and transmission shift cable. The 12 bolts that connect the subframe to the intermediate frame were removed. A low
4-wheel dolly was positioned under the frame. Wood blocks were stacked on the dolly so there was about a ¼" gap between the wood and the frame. The front of the frame was supported with a jack while the 2 frame-to-body bolts were removed. The jack was then lowered and frame dropped down the ¼" so it was clear of the body. The frame will slide in the side rails as it is
pulled forward. Have wood blocking under the rear of the frame so when it comes out of the side rails it will be supported. Joe Bertrand 

I removed my front frame with complete drive train today, here is what I did.
1.   Fill air bags.
2.  Put jacks stands behind six frame bolts where frame will separate
3.  Raise to about 23 1/2 inches to bottom of front bumper.
4.   Remove grill.
5.   Remove valence under grill.
6.   Remove bumpers from frame.
7.   Disconnect front and rear batteries.
8.   Remove inner fenders.
9.   Open pet cock on radiator and drain.
10. Disconnect radiator overflow hose from overflow tank.
11. Remove battery.
12. Remove rear battery cable from top of cross member & solenoid
13. Remove air conditioner compressor, it will hit body support
14. Drain freon and disconnect hoses from evaporator housing.
15. Disconnect hoses from windshield wiper motor.
16. Remove battery tray.
17. Disconnect oil filler from top of grill opening.
18. Disconnect air hoses from compressor & remove compressor
19. Remove hoses from steering box.
20. Remove steering box from frame and tie-rod end.
21. Remove lines from master cylinder and plug hose in master cylinder.
22. Disconnect rear brake line from brake block behind driver-side tire,
       and remove electric wire.
23. Unbolt rear brake line from frame.
24. Disconnect bracket holding gas line on cross member.
25. Remove rubber hose on gas fill line.
26. Remove bolt on emergency cable at Y fitting and pull cable
       through frame to front.
27. Unbolt gas vent hose.
28. Remove rubber gas hose behind cross member; plug hose.
29. Remove gas tank to canister hose on top of cross member.
30. Unbolt gas tank vent line from cross member.
31. Open engine cover; remove bolt, holding two cables to cruise-
       control transducer.
32. Remove lower cable from transducer and vacuum hoses.
33. Remove air cleaner and disconnect two water hoses to rear.
34. Disconnect wires for alternator, oil switch, water switch, distributor,
       starter (2), cruise control, air conditioning.
35. Remove throttle cable and vacuum hoses.
36. Remove transmission shift cable
37. Remove oil filler tube to engine
38. Remove dipstick to coupling
39. Loosen gas fill tube from body
40. unload torsion bars
41. Support front of frame & remove 1 2 frame bolts
       I used an engine lift on front cross member to hold frame
42. block rear wheels
43. remove 2 front body mounts
44. when pulling frame forward lower frame so steering box mount will
       not hit fiberglass
45. remove wheels
46. To pull frame forward I backed my truck about 6 foot in front of the
       GMC & used 2 come-a-longs from the rear bumper to each side of
      the GMC frame Bill

-------------------DIPSTICK REPLACEMENT
Anyhow, with time to relax and listen to the wind in the pines and the thunder approaching, I decided to check the oil.  NO reading, but it felt funny going in and I suspected the bitter truth.  Threw a quart in just to be safe, and after making our way home the next day, I confirmed my fears: a rusted-out broken dipstick tube.  A call to Cinnabar got a new lower tube on the way (~$14 + shipping), and I set in to pull the old one.  After several false starts, this is what worked for me:

  1 Remove left wheel, wheel well, and exhaust manifold for access.

   2  Take a large (10") pair of Vise-Grips(R) with good teeth on the ends of the jaws, and drill a 1/4" hole in the body of the tool about 1/2" behind the jaw pivot rivet.  (Avoid jaw, springs, etc.)

   3   Clamp the tool tightly on the broken dipstick tube.  Go low enough to get solid metal to bite.

   4  Using a small slide hammer with a hook end (J.CWhitney Dent Puller, about $25 a few years ago), hook into the hole in the Vise-Grips, align as straight as possible, and slide-hammer the sucker outa' there!

   5   Select a crow's foot socket that just fits over the new dipstick tube.  Slide it down to the expanded collar of the tube, insert an extension bar, and use to hammer the new tube into place.

    "I love it when a plan comes together!"    HTH.Rick Staples

My advise would be to push the stub on thru. Run a long piece of bailing wire down thru the hole where the tube was and out the oil drain hole. Did I mention that you need to drain the oil?? Then with a small punch push the stub on thru and it will slide down the wire and into your hand. How's that for an alley mechanics trick.<G>....................Terry

>Good time to start up an old discussion...... This might be an urban myth on the GMC.  The problem is usually bad battery cables, grounds and batteries, not the starter..... I have not seen proof that this exists on the GMC.
Read here for grounding problems

  I would agree that this is a myth.  I have no such shield on my starter and today I stopped for lunch at 100 + degrees and the GMC had plenty of starter power on HOT! restart.  Additionally, I have headers and had just driven it over the San Jacinto mountains and it was hot.  I think people with this problem have electrical problems as you have described. A heat shield on the starter shouldn't be necessary and is not addressing the real problem.Phil Swanson

I also had a hot start problem. Had  the starter rebuilt (rebuilder said it wasn't in bad shape to begin with) Ran a new #2 ground wire from the battery to the motor and replaced a group 27 battery with 650 cc amps with a new group 24 battery with 1000 cc amps. No more hot start problems. Don't know which of the above fixed it (?) I also bought the heat shield for the starter and its been in the blister pack on my bench for over a year. Dick Missett

Was the throttle wide open during this test - it should have been.
I like to remove all the plugs - that way the engine spins easier and
faster - pull out the plug to the HEI.

But a compression test is not the final say - just a guide to engine
condition - What you should do next is a "Compression leak down test /
differential compression gauge". it's only slightly more complicated.

here's how I do it.
I beg or borrow a leak down tester from good machine shop or speed shop -
I give then a deposit which is equal to the replacement value for their
tool - just in case.

. do NOT remove all spark plugs
. remove the connector from the HEI
. remove the distributor cap
. stuff a rag in the oil fill neck
. remove the pvc valve
. remove air cleaner

. mark the base of the distributor with the location of each plug wire
. turn the ign key to position the rotor to just a wee bit past a plug
  wire position (this will be about TDC for that cylinder) & both
  valves will be closed.
. the leak down tester needs an air supply input of about 120-140 lbs.
  each cylinder is pressurized with a known amount of air pressure and the
  second pressure gauge indicates the amount of leakdown (lower pressure
  thru a standard orifice build into the gauge). The differential on our
  engines should  be greater than 80% (source pressure vs leakdown
  pressure) Brent Covey - correct me if my percentage is too low.
. I leave the other spark plugs in  to help prevent the engine from
  spinning due to the 120lbs of air pushing on a cylinder.
. Now here's the good part -
  listen at the valve cover PVC hole for air (blow down from the
  listen at the tailpipe for air (exhaust valve leak)
  listen at the carb (intake valve leak)
. with only 70k on your engine - you may only need a valve job

hope this helps
Been there (a number of times)done that  (more times then I care to admit)bought the tee shirt Pete

You can get the 1972 Eldo. front engine mount, and the right hand shaft
and bearing support from Maximum Torque Specialties  http://www.mtscadparts.com/
Jerry Lader


The carb number is on the left side of the carb, on the flat surface above the throttle shaft. The number will run vertically, and there will also be a number which is the day of the year and the last digit of the year it was manufactured.

73 & 74, 455's     7043254

75 & 76, 455's   Federal  7045254   Calif.  7045554

77   403's    Federal   17057254     Calif.  17057559

78   403's    Federal   17058254      Calif.   17058559

the parts list for the Quadrajet carb used on the GMCMH.To view, go to


Bob Drewes   seSD

Carburetor Rebuild

 I had mine rebuilt by A & G Carburetor in 1996, which is located on the West side of Chicago in Oak Park.  They told me that after their overhaul, all I would have to adjust was the idle mixture and idle speed.  We mounted it back on the engine and never made any adjustments to it.  I had stripped the bowl threads for the filter nut and they replaced the bowl.

The rebuild included epoxy sealing of the bowl plugs, leather accelerator pump, needle and seat (plus a spare needle and bowl gasket for your tool box) and replacement of the choke pull-off.  They also check the throttle plate shaft for wear/leakage and bush it if needed.

I sent it via UPS Monday morning and had it back via UPS Friday afternoon that same week.  The carb came back so clean I called them and asked what they sprayed on it to make it look so new.  Their response was they use an eight step washing process and nothing was sprayed on it.

Not knowing of them other than seeing their ad in the GMC Motorhome Marketplace magazine back then, I called and talked to them 3 or 4 times, to satisfy myself that they knew what they were doing. Each time I was more impressed than the previous conversation.  There was no BS, if you know what I mean. They certainly should be able to tell if your card has the right components for the Motorhome, and if not bring you up to speed.

Back then they charged $138.75 for the overhaul, $15.89 for the choke pull-off, and $40.00 for a replacement bottom bowl. Call them and talk to them - 708-386-9804.
   Paul Bartz

Carburetor Throttle Adjustment

 You can adjust the final throttle cable travel by simply bending the accellerator pedal linkage under the dash.    Yes, just bend it.  While I removed the pedal assembly to fix mine some years ago, you can do it in place without removing it  after you unhook the cable from the pedal (hindsight).  There is no reason to buy a new cable.  I also had the same prob with the cable/bushing end at the carb = nonexistent  - got a nylon bushing close to the same size at the local hardware & massaged it so it'll fit. Ritch  & Betty
"You can also adjust the throttle slack by just using a small fishing line split lead sinker.  Crimp it on the throttle cable just above where it connects to the pedal.  It will take up some slack on the cable.  If really bad just use two sinkers.  You might find this easier than trying to bend the pedal the right amount."Emery

GMCs are known for the gas peddle hitting the floor before the carburetor is at wide open throttle (WOT).

Sometime, remove the engine hatch and look down the carburetor throat and see if the "big boys"  open up when your gas peddle is WOT. (you will have to open the choke flappers.   Probubly not, so here is what I call the Emory-split-shot-fix for this problem

There is so much slack in the acellerator linkage, it might just be a good idea to do this mod (just in case).

I did the same and then heard that Ken Frey sells a replacement.  IIRC, the
nylon bushing should be 1/4" ID.  I put it on a 1/4" screw between two
washers and tightened down a nut on top of that.  Chucked the screw into an
electric drill and spun the bushing on some 300 grit sandpaper.  After maybe
10 minutes of the sand/fit cycle, I had a properly sized bushing.Patrick


I had a problem that my 1978 GMC MH engine (403) would die when I exited the interstate and arrived at a stop light. It would start up ok and run fine until the next stop sign and die.
I discovered that the vacuum operated throttle lever actuator was leaking air. Its purpose is to increase the idle speed when the A/C is on.
A simple bypass is to plug the vacuum line and increase the idle speed about 200 RPM's, but the increased idle sometimes leads to dieseling when ignition is turned off.

After weeks of searching for a replacement I was eventually refered to a place in Texas that rebuilds them.
REBUILDERS SPECIALTY INC. 15049 Valleyview Rd. Forney, TX  75126 972-564-4141
They were very helpful and rebuilt my Throttle lever actuator for $15 (just like new).  Much better deal than finding another old unit with aged diaphragm. John Wolever


I always tell new owners that if the lines are 10 years old or of unknown age, replace them. Lines that look good break all the time and can cause
disastrous damage to the engine within just a few seconds, sometimes accompanied by a fire when oil hits a hot exhaust manifold. See:
Bob Burkitt

J.R. Slaten who is a GMC owner makes and sells the stainless steel  braided hoses for both the 455 and 403 engines. They are first quality, bulletproof and should never have to be replaced ever again. He is a member of the GMCNet and you can reach him at jrslaten@aol.com and his phone number is 502-363-3011.J.R. Wright

 Brent Covey wrote

Theres a few things to pay extra close attention to-
All carb kits and parts within them are ordered by looking them up relative to the carb number stamped into the carb body.  This number will start out with "70xxxxx" or "170xxxxx" and you'll need that to order the goodies for the rebuild.
Most GMC carbs will have very little wear as this is something that occurs more in high city traffic mileages, not just zooming down the highway.
Every time you encounter a black coloured foam float, replace it. Some GMC's use a brass one which you can shake and see if its pinholed and allowed liquid inside. Most are fine.
ALWAYS buy a new vacuum break diaphragm, this is the little bellows thing that pulls the choke open, located at the passengers side front of the carb. These are the #1 failure in carbs, and cause flooding and hard starts when they fail. A used one is OK to use, but make sure you have a spare anyhow, and carefully check that it holds a vacuum and pulls the choke open when
vacuum in applied. 

Never remove the throttle plates or shafts or choke plate/secondary air valve unless you are replacing them, as the screws often break off and are in a position if they fall out the engine will inhale them, which is bad news.

The vacuum break adjustment is altitude sensitive. The kit instructions do not make it clear how its adjusted, but the essence is, it should open about 3/16" when the vacuum break diaphragm has vacuum applied to it. This is approximately the distance the little groove embossed into the choke flap is wide. Once its on, you may need to fiddle a little with this setting, it needs to open less at low altitudes than higher ones.
Main jets need a PERFECT fit screwdriver to remove them. They can strip and tear up with the wrong one, so get a really tight fitting screwdriver for them. A sharp blow to the handle of the screwdriver with a light tool will usually aid in breaking them free, they like to get stuck in there.
Retain the original GMC main metering rods, they're perfect for the GMC. Jets can be changed to increase mixture strength a little, all GMC's should  use at least a '70' jet, and some could benefit with as much as a 74. Basically as a rule of thumb go 3-4 jet sizes richer than whats stock and you should be OK. At altitudes over 3000', #71 is fine for most people. At 5000'+ 70 will do. Jets have a part number ending in a two digit suffix that designates their size, a 0.070" opening in a jet is a "XXXXX70" jet as example. The prefixes are determined by carb family, and can be looked up in the book, jets are about $3 each from GM.
The secondary air valves are opening too soon on most carbs, you should have a close look at yours and make sure you understand where the adjustment is located. Theres a set screw beneath the carb top, upside down that locks the screw visible inset in the passengers side rearward edge that adjusts spring tension. Double check lockout operation, not solely for locking out, but also for releasing the air valve again as well.
Float settings aren't especially critical, go with the book on those. Be sure the needle is hanging off the rearward edge of the float arm, not hooked thru the holes in the arm which many people do. Be sure to stretch the float hinge axle (C shaped heavy wire thing) open a bit to ensure when the carb top goes back on it compresses the axle enough to ensure the float has a good fulcrum to seat the valve with.
Make sure you install the choke seal and plastic hollow pin between the housing and carb body. The choke must be adjusted so that it is strong  enough to just close the choke well at room temperature, you need to slightly open the throttle for this. Check that the fast idle cam behind the choke is pulled to the top step when the choke is closed and throttle is released.. Adjustments of the choke spring are best carried out with the plastic choke housing cover loose enuf to turn with your thumbs, and the screws out. The apparent screwdriver slot in the plastic cover will break if you attempt to use a screwdriver usually.

The plugs in the bottom of the carbs can work loose, and this usually manifests itself as high fuel consumption, and very slow starts after youhave let it sit a few hours with a hot engine. You can inspect them for signs of leakage, usually they're OK, and a dollop of some non-fuel-soluble sealant will keep them dry usually. You can fix a leaking plug by judicious restaking of it, but if you drive the plug too far in, you'll possibly block the passage its for. The plugs are self evident from their grey colour.

Be sure to double check screw tensions just before you install the carb as well.


Its imperative of course that you take your time and have very clean parts to work with. Invest in some carb cleaner and let it soak overnight. Extremely stubborn deposits will usually yeild to Oven Cleaner, but this will eat the castings fast if not supervised, washed in water thouroughly, and de-activated with acid, such as white vinegar. You MUST pay close attenbtion if you use a caustic cleaner, and be sure you've soaked it in vinegar etc very well after, and rinsed in water.
WD-40 makes a reasonably good source of 'compressed air' for cleaning passages. The engine will also start and run OK on WD40 if you want to prime it a little thru the bowl vent. Whatever you do, make sure its clean as you can get it, before you reassemble it.

Most carbs will look pretty good and only need a kit and slight tweaking of the adjustments.

The big fuel filter nut is 1" and must be tightened carefully so as not to strip the threads. When you reconnect the smaller fuel line to it, use a 1" wrench (many crescent wrenches will reach 1" satisfactorily) and bend the tube slightly in the direction of 'loosening it' while you tighten the flare nut on the end the last bit, which will put some tension in the direction of 'self tightening' in the tube and help keep it tight. You will need a flare nut wrench to loosen the fuel line, buy a TOP QUALITY one, the cheap ones are as good as useless. If you just cant get it apart, taking it off in the first place, Vise Grips will usually do it OK, or you can cut the line and use a six point socket to get the flare end out and replace the steel fuel line later. Dont re-use a steel line with a damaged flare, rounded off or manhandled nut, or a kink in it, its a fire risk.
Set Idle mixtures once the new carb is on by 'lean roll'. Test secondary airvalve operation, and adjust as neccesary, and next day on a cold engine, check the cold starting and fast idle, and tweak if need be. 

Thats basically most of it. I am sure I've forgotten something I'd have thought of if I had one right here, and of course, if you see anything awry, find out what you need to do.


As always, any carb or fuel system component needs doubly careful assembly as any leak is a terrible fire risk. Be extra careful when you do this sort of work and make CERTAIN that if you smell fuel you stop AT ONCE and investigate it. This is doubly important now that the Reformulated fuels with heavy Oxygenate content are causing deterioration of soft carb components at a high rate, and much faster than we were used to in years past.
Good luck with your QuadraJet, you should be able to do a very good job at home if you take your time.
Brent Covey
Vancouver BC



 After getting the aluminum radiator tested and on the market, we decided to test out electric fans for our the GMC.  We have heard good and bad results using electric fans so we went into it with an open mind.  After almost 2 years of testing we are still on the fence.  Some things we have found out are:

1. Don't put an electric fan in front of the radiator.  It ends up blocking the air flow to the radiator as a "pusher" fan.
2. You need some big fans to get the required air flow.
3. A good shroud design is critical for the electric fan.
4. A good fan control system is mandatory.
5. If you don't have the entire cooling system in first class condition, forget the electric fan.
6. Forget the auxiliary fan, if the fan clutch is working right it will just get in the way.
I am still working on fan blade design, shroud design, and fan controllers but maybe one of these days----.
I have found the only time my fan comes on is when I am in traffic (under 25 mph), on a long steep hill or mountain road, or just idling the engine.  On the highway, the temperature stays normal.  We are getting close, but still need a little more design improvements to be acceptable.Gene Dotson (5/31/07)

J.R. wrote: I find that when I get over 35 mph my 2797 hayden pretty much freewheels and uses little hp.
 Agreement here!  I ran a modified 454 in a '55 Chivy for 8-9 years and went through some real cooling wars.  I tried everything from flex fansto twin( "16 "tornado") electrics and nothing worked when needed most. In fact, the twin electrics would over heat at 65 mph because they blocked too much ram air.  When those babys kicked in in traffic, (when engine rpm is also at it's lowest) they pulled 40 amps on start up and 20 total to sustain. The turn signals slowed down & the lights got kinda dim also.  I eventually wound up with a clutch fan and an after market shroud that really worked well.  No matter what the conditions, the engine temp never exceeded 190 deg.   I HAVE used  electrics with good results in some applications though. Understand that any electric will pull better than it will push.  I have a daily driver '57 Chivy with modern A/C and with the stock radiator it
has a tendency to get hot in traffic.  I have a similar shroud & clutch fan on it as on the '55 however, it is mounted inside the shroud and is relay activated off of a signal from the brake light switch.  It works well when needed (stop signs etc.) and since I have a 88 amp alternator on the vehicle, the bad effects of the amp draw are minimal.

I'm on my 3rd clutch fan in the GMC.  The first one was not the recommended Hayden 2797, I hadn't started reading the digest at that time so I made the mistake on my own.  The second one was the recommended heavy duty Hayden and it solved all my problems.  It just crapped out in 1,000 miles. I replaced it under warranty and it works just fine now.  The big difference in the heavy duty vice any other is the percentage of engine rpm that the fan spins at when not locked up. This is exactly why the first one didn't do the job. It spins much faster in the neutral position and is exactly what is needed to cool the GMC under normal driving conditions.

 I hope this help anyone thinking of switching to electrics for primary cooling. Steve Ferguson
We replaced the motor driven fan with an electric mounted on the back of the adiator. It is a high flow, low profile fan. We modified the existing fan shroud with fiberglass and used that as part of mounting. It has worked quite well thus far, with no overheat problems. We also feel it is A LOT quieter and leaves a lot or room, making servicing and belt replacement easier. I believe this is something many recommend against, (why? I don't know) but it has worked well for us. We haven't kept track of mileage lately, having come to accept the fact it will never be all that great, so I can't comment on that.We like it just fine! Tony Bad
I put on a 15" fan late last year. I took it off a short time ago and put a new fan clutch on with the old fan and shroud. The engine temp got up to 230 degrees in traffic at about 90 degrees outside temp. I also subscribe to the opinion of someone whose post I read here, that the added amperage draw of the cooling fan is an unnecessary added load on the alternator belt arrangement which is inadequate already. David
Electric Fans have been used with marginal success.  Most people that I know that have tried them have gone back the the factory setup.  Hard to beat that big fan and shroud for moving air when you need to.  You will also need to beef up the electrical system to handle the extra load of when the fans run.  This could be up to 40+ amps of surge load depending on the size of the fan or fans.  Remember that your also running your AC system which is already a large current draw.  Also means that you should probably go with at least a 100 amp or larger alternator and you will have to upgrade the wiring and battery isolator or use a combiner of a higher rating.  Not saying that you can't do it.  Not inexpensive either. J.R. Wright

Fan Clutch

The left column is engine rpm with the fan clutch disengaged, engine rpm and actual fan rpm.  The right column is the same, only with the fan clutch engaged.  I tested two Delco 16-4644 units, which is the recommended replacement for our application, and you can see that they spin at a very high % of engine rpm when disengaged, and nearly lock up at engine rpm when engaged.  I'm sure that at higher rpm there's
more slippage.
 Notice that the Hayden 2797 and the AC Delco 15-4949 both are the same units.  I checked these several times as I didn't trust the results.  I don't think they would work well in a warm location as they spin at less than 50% of engine rpm when engaged.

All testing was done using a 1hp electric motor driving a water  pump with a 7 blade, 3" pitch 5,5" blade fan.

                            AC Delco 15-4644 #1

         Disengaged                                    Engaged
 Engine rpm            Fan rpm       Engine rpm         Fan rpm
    1,292                  1,000         1,268                  1.238

                           AC Delco 15-4644 #2
   1,289                  1,030          1,259                  1,227

                          AC Delco  15-4949
   1,320                   357            1,255                  1,226

                          Hayden 9727
   1,317                   650            1,252                  1,227

 Notice how close the numbers on the AC Delco 15-4949 and Hayden 9727  are.  That is because they are both made by Hayden.  9727 Fan clutches  sold under the name Hayden are made in the USA.  9727 Fan clutches  sold under the name Torque-Flo are made in China. SteveF

How to change the Fan Clutch by Richard Walters

Cinnabar sends this one and the quality and performance is very good.
#15-4644--------------This is the Delco #

 #12529772-----------This is the GM Part No.
$68.19.  Boyd


There have just been too many problem with this product..  I put on three of them in a two week period, and then put on the Delco tha worked first time... gene



I have been using a Hayden 2797 fan clutch which I purchased 3 years ago at a Pep Boys store in Texas for $69.99.  Returning from my last trip (Prescott) I didn't think that it was functioning as it used to. I recently removed it and found some fluid seepage around the hub.  I returned it to the local Pep Boys and they replaced it with a new one under warranty.  By the way, the new one had a price of $64.99.  The parts man opened the box and the size and shape of the unit looked identical to the one I brought in with me.

Upon getting it home, as I opened the box, I happened to notice an arrow pointing counter-clockwise on the face of the clutch with the letter "R".
Since the 455 engine rotates clockwise (viewed from the front), I thought this strange.  I called the technical line for Hayden, 1-800-433-7508, and spoke to a technician.  He confirmed that the correct clutch for a motorhome with a 455 engine was their severe duty 2797.  BUT, he said that it should have a clockwise arrow with the letter "F" for forward.  The letter "R" means reverse rotation and he felt that what was in the box was actually a 2784, reverse rotation clutch and he told me it would not work on a 455 engine.

I returned to the store and they opened another 2797 box and it did indeed have the clockwise pointing arrow with the letter "F".  Either the factory or the store had put the wrong clutch into the box labeled 2797.

So, those who have or are considering using a Hayden 2797, be sure check to check that you have the right clutch in the box or mounted on your GMC.Emery Stora

  The OEM fan clutch was made by Eaton Corp, where I was employed for 23 yrs.   A silicone fluid clutch will never lock up.  In fact the fan speed will peak out at a predetermined speed and stay pretty constant regardless of the input (water pump) speed.  Of course as the slip speed increases, more heat is built up and the internal temperature rises and actually drops the fan speed a bit.

Also, there was some discussion about how to store the fan clutch off the engine.  The best orientation is nose down, but you need to be careful not to damage the bimetalic coil at the front of the unit.  If you store the unit shaft flange down, the silicone fluid will seep into the ball bearing and dilute its grease and eventually leak through the seals.  Storing the unit with shaft horizontal would be fine, but it's a little awkward.  Unless you lose some of the silicone fluid, the orientation of storing should have no detrimental effect on the unit's performance.

By the way, the most common failure mode is bearing failure.  There's a sealed single row ball bearing with a special, high temp grease.  After a lot of use, the grease tends to dry out and then the bearing bites the dust.

The roar sound on start-up means it's working.  Typically, the silicone fluid drains down inside the unit while it's standing still.  There is a simple pumping mechanism inside that pushes the silicone fluid into the reservoir (front) when cooling is not needed.  As that happens the fan speed goes down to an idle condition of around 500 RPM, even when the water pump is turning at high RPM.  As Emery said, when the radiator heats the air to a high enough temperature to activate the bimetalic coil, a valve slides open to allow the fluid to fill a set of concentric, close fitting grooves between a disk that's attached to the input shaft and the "body" that the fan is attached to.  The grade or viscosity of the silicone fluid determines how much torque it will transmit and therefore the speed of the fan.  An ingeniously simple but effective device.

Someone mentioned hearing the fan come on when entering a freeway ramp.  It also happens frequently when the engine idles for a while.  Basically when there isn't much air flowing through the radiator, the temperature at the bimetalic coil goes up and engages the fan.  Then when you start moving, the temperature returns to normal and the fan cuts back out.  After that it should come on and go off based on ambient temperature and engine load. Clark
I took a poll of my GMC buds, and asked if it was possible to change clutch without removing the shrowd and belts on my 1977  455.

2 said no
1 said maybe
2 abstained by not being home.

But  yes Virginia it is possible.

I think it can be done entirely from the top but I took out some of the bolts from the bottom.

I had to remove the fan from the clutch inside the shrowd to remove and replace the clutch. The one I removed was a Delco but it did not roar at start up , and did not operate on the last two hills so I replaced it. It took me two hours, but had I known this was possible I think it could be done on a cold engine in about 30 Min. Gene

Radiator Thermostat
Robert Shaw #330-195 balanced flow thermostat 195 Deg.

I think you will find that the Stant Thermostat, even the Stant #65359 "Superstat" 195 degree "heavy duty thermostat" will not hold up in the GMCMH application.  Several GMCers have found out the hard way & lost their engine do to over heating caused by a Stant Thermostat failure which shut off the flow of water.  One of the two support legs will break & close the water flow path.  I would not run with anything other than a Robertshaw thermostat, 180 or 195.  Available at local Auto Zone parts
house, ~ $5.  If you already have it installed, I would remove it & install a Robertshaw...seen several GMCers that have been there with undesirable results.  Duane


> Try partsamerica.com for Robertshaw thermostats.  Look in the cooling section.  They have the 330-180 and 330-195 that works in our units.  I prefer the 330-180. Gene Dotson
Right on, Gene!
PartsAmerica also sells there stuff thru local stores.  In this case Kragen and Schucks... which is just down the street and theoretically has one in stock, according to the PartsAmerica web site. 
They sell them under the Prestone label.  Prestone 330180, to be exact.

Parts Amercia is the same as Kragen/Schucks.  They used to sell this thermostat under the Prestone 330-180 p/n.  Seems like they sell Mr Gasket's version now.


Thermostat housing Gasket

Radiator Cap
The 9# cap is available at your local GM dealer.  Ask for AC Delco type
RC32 part#6410619 and a 10# cap is available at most autoparts store
under a Stant part number 10237, if they don't have it, have them order
it for you. The lower pressure rated caps are not carried by the store
much but they are available.J.R. Wright

One item that makes the radiator leak is using the wrong Pressure Cap.  Should be 9 pounds (no greater than 10 pounds).  The 9 pound ones that I have seen  are round & hard to get off & on.  Therefore I use a 7 pound cap for many years.  The high pressure caps will expand the Radiator & develop leaks.

Also your coach is running much...much to hot & this will kill a Radiator also.  The OEM temperature gauge should never get above the 3/8 mark if the fan clutch is working.  Many fan clutches do not work properly.....you should hear the clutch engage before the 3/8 mark & stay on until the temperature drops.  I use a Hayden #2797 (severe duty) from Pep Boys (about $90 Lifetime).  If it works right it only makes noise when the temperature is up & first thing in the morning.Duane

Temperature Sender
This may be old news for most but doesn't hurt to be told. The original
temp. sender on all GMC Motorhomes at :
1/4 is     225 degrees,
1/2 is     250 degrees,
3/4 is     270 degrees, and
H is        280 degrees.
This has caused a lot of engine damage when engine starts to overheat. Go to NAPA and get a
TS-6469 sender and it will show:
1/4=      180 degrees,
1/2 =     215 degrees,
3/4 =     240 degrees, and
H =        255 degrees.
Since the standard pressure cap raises antifreeze solution boiling point to 250 degrees this will be
more accurate readings. I don't know why they did this but it is a
serious problem. Info from GMC Motorhome News published by Cinnabar,
Dec. '95 issue. I did this and it agrees exactly with my VIP digital
readout. bill

Oil Temperature
A few years back I also mounted the sending unit for an oil temp gauge in the oil filter adapter, measuring the oil temp leaving the engine.  I also found my oil temp ran around 260 degrees F with a normal engine load on my 455 engine in a 76 Royale.  I became very concerned an spent a lot of money for an auxiliary engine oil cooler.  At the same
time I put on several "Westac" gauges.  They mounted the sending unit for
the engine oil temp in my drain plug (only charged 5$ above the normal
sending unit price).

I calibrated all my temp gauges in cooking oil before I put them in.  What I found out was I spent a lot of money for the auxiliary oil cooler that I probably didn't need.  I still have the
temp gauge in the oil filter adapter and it normally runs from 40 to 60 degrees F higher than the oil pan temp.  I also have another oil temp gauge in oil filter adapter measuring the oil temp coming from my auxiliary cooler. During normal operations the oil is cooled about 40 to 50 degrees F going through the cooler.  My oil temp measured in the oil pan drain plug never gets more than about 15 degrees above the engine coolant temp under max load conditions.

Based on my experience with an  oil temp sending unit mounted in the oil filter adapter, your 260 degrees F coming out of the engine is about normal.
Chuck Aulgur

Water Pump
The correct water pump has a cast iron 6 paddle blade of 4 1/4" diameter part # 412265-D24. It must have the D24 on it to be the correct one. They can still be obtained from Ray Curtis at 1-800-764-3673 for about $50. plus core.

Speed Control
 Remanufactured transducers .  2 year warranty,  part number  25030251,
$ 85.95 exchange plus  $4.00 shipping- costs $4.00 to return core for $4.00 credit so don't bother. Total will be $104.00 on Visa card so there are no COD charges.
United Speedometer & Instrument, 2431 University Ave,  Riverside, CA  92507,

Checker Auto #36-102 ($60.99 plus core). Checker lists this one for the '76 Caddy Eldorado and the number cross references to GMC #2503 0712, the number shown in GMC dealer's computer as the successor for the one listed in our GMC Motorhome parts manual.  The AC/Delco rebuilds have been discontinued. Cinnabar also says they are unable to get rebuilds, so I was happy to find one at Checker. It's rebuilt by A1-Cardone.Richard

Micro-Tech Automotive  no longer supplies transducers

Electric Fuel Pump Wiring
Click For Detail Click on picture
The circuit above will allow the pump to run during cranking or only when there is oil pressure.

My question is, should I bypass the mechanical pump altogether, remove it and block off the opening, or can I run the electric pump together with the mechanical?  If I run them together I would only use the electric as needed. Don

Most owners leave the mechanical pump on.  It is possible for the diaphragm to leak into the engine with or without the electric pump.  (see the note farther down )*

One "booster" connection for the electric pump is to power it on with the gas tank selector switch so that the electric pump only comes on with one or the other tank.

Here is the "Carter" safety wiring for electric pumps.  click on picture.
you can power it from the center terminal of the diode isolator as Ken says.   Good time to test your isolator to see if it is working and if you remove the isolator someday you will have to rewire.

  The pressure switch listed on the
Carter diagram on your site is available from several internet sources --
for about $25.  The ACDelco G1809 and Standard PS64 are basically the same
and sell for $8-13 at Rock Auto, Autozone, OReilly's, Advance Auto, NAPA,
etc.  Just ask for a 76 Vega oil pressure switch with 3 terminals.  Carrying
a spare is not a bad idea -- Ken H.

Good time to mount a filter by the electric pump and remove the filter from the input to the Carb.

Use only the best rubber hose.   It has been suggested to use injector hose because it lasts longer.

You can run the electric pump all the time ---- and why not ?-- one thing less to remember when you are having problems with vapor lock.   Keeping it simple is a good idea. Fewer connections, hoses, components, etc. 

* This year (2005) there was a suspicion that a bad mechanical gas pump destroyed 3 engines.  The first was the OEM engine, the rebuilt engine used the old pump and lasted only a short while.  The next rebuild also used the same pump and lasted only a short while.  This time it was determined there was a small leak in the diaphragm of the pump which was pumping gas into the crank case.   This does not happen often, but it does happen and may be hard to detect.. 


Electric fuel Pump

The model # is Carter P4070

Summit Racing had it for about $56, . I bought mine at a local speed shop for $65
here in NY. Tony

I run the Carter 4070 on our GMC. I also have a fuel pressure gauge mounted on the outside of the windshield. When I first installed the electric pump I put it after the selector valve so I could opperate it with either tank. What I found was I had between 5 to 6 pounds of fuel pressure using the elecric and mechanical pumps. When I shut off the electric pump the fuel pressure would drop down to 3 pounds. With the electric pump bypassed I was back up to 5 pounds.
 I remounted the electric pump before  the selector valve and connected it to the reserve tank only. That way if I need the electric pump I just switch to the reserve tank. This way there is no drop in fuel pressure while running on the  main tank which is what I do.
 Anyone looking for an oil pressure switch that will shut the electric pump off, go to any auto parts store an ask for a oil pressure switch for a 1977 Chevy Vega. This is the one to use and is cheaper then the one from Summit Racing. I don't have the part number. Just tell the counter man you want the one for a Vega. Jim Wagner Brook Park, oh

After checking out the suggestions made I determine that the pump pressure was to high 8 1/2 PSI.
Fortunately I did not find the pump that I was after at any of the three parts stores in town. Did some more checking at the pump and found the positive and negative reversed. This turned the pump in the wrong
direction. Switched the wires and the gas line hoses and now it works fine. My eyes are no longer what they should be. What I have learned is that the pump puts out more pressure in  reverse   Frank
I mentioned the surging on long grades. Somebody said it might be starving for fuel. I thought I should be able to see that on my fuel/air meters. I  left the electric fuel pump off today. Came to a long grade. Stepped on it real  good---. When the secondaries kick in the meters go to rich and stay there. Sure enough close to the top the meters went to normal then lean and the lady began
to surge. Hit the pump----meters went rich and the surging stopped. It is neat when your toys confirm what you think might be wrong.Arch

GMC Exhaust Systems

You got-em, smoke-em.  If you have good exhaust manifolds, resurface them, remount with copper gaskets. The best fit with a copper gasket is from Lenzi http://www.bdub.net/lenzi/index.html#CopperGaskets ($55) Retorque them some time, if you think of it.  They worked for 30 years maybe they will work a while longer.  If you want them to last longer on our crappy gas, put in an A/F gauge and
get your carburetor fixed until the exhaust does not run lean and hot. When the manifolds crack, go to headers.

When you cannot find manifolds, put on Thorley iron headers.  Don't coat them, it cost two times as much, burns off any way, and probably does not do much for reducing the radiated heat, and cannot be proven to do much for horse power unless you have a dyno (which we don't, and the gain is so little who cares).  Put them on with copper gaskets and torque to 35 ft lb--two or three times after running. The best fit copper gasket for headers is Mr. Gasket http://gmcmotorhome.info/engine.html#copper .($40).   Keep your old mufflers if they are ok, you will only gain 10 horse power if you replace them with a high flow muffler and a three inch exhaust anyway, and that is not enough to spit at. Never end a sentence with a preposition and add some fans to draw the heat out of the engine compartment when you stop or put vents in the wheel wells..

When your old mufflers burn out, replace them with straight pipes and cut in a single three inch turbo muffler in the rear (or use two up front doesn't really matter).  Keep your 2.5 inch exhaust pipe until it burns out, you will only gain about 10 horse power if you replace it and that
will come too soon anyway.  It is a little louder with a single muffler but there is little back pressure.

When the 2.5 inch exhaust pipe burns out, replace it with a three inch pipe from the 2.5 inch Y straight pipes to the three inch muffler in the rear. You will help the headers do their job and you will pick up a few horses.

When you have all of the above, you have headers that won't crack, one less muffler to worry about, a full flowing system and you may have picked up 10 to 20 horse power.  Not much here to stress over.  Do what you have to, this is not going to change your life.



Some have reported success by drilling an 1/8" hole in the head, down from the top into the bolt and putting Kroil in the hole.  After taking off the header, I weld a nut on the bolt stub and let it cool for an
hour.  Then try to turn it both ways.  Then smack it with a hammer on the end, try to turn it again, then break it off, drill it out and tap the hole. Gordon


  I ordered mine locally and it took a week to get them. $50 for MR. Gasket
P/N 7170. Jegs (800-345-4545) lists the same for $33.99 (P/N 720-7170). HTH.Nate
  The easy way to work on them is to remove your wheel well Liners.  Just remove the screws at the wheel well edge and there is one screw in the middle top inside the wheel well.  For more room you can remove the wheel from the side that your working on.  This puts you right at the manifold.  If you have a 403 in a later model 77-78, you can get them from the top.  If your using the stock exhaust manifolds the only gasket I recommend is the FelPro 1439 gasket set along with the  High Temp permatex copper gasket silicone sealer.  For a little bit more you can get the solid copper gaskets that should last forever. J.R. Wright

Over the past 13 years, I've had to  do the manifolds three times.  Each time I had them resurfaced and then put back with gaskets.  This past spring another gasket began to leak.  When we got in there we also found a valve or two going bad so I had a valve job done too.  Had the heads as well as the manifolds resurfaced.  Put them back without a gasket and everything works great.  Noise level is very low and I was able to make a trip to Southwest Texas with the heat reaching in the high triple digets without overheating while running the AC.  I think if you are using manifolds, this is the way to go.Justin
One of my first projects was to replace both manifolds with brand new ones from Cinnabar.  One side, the exhaust port sealing surface was in relatively good shape, so I installed the new manifold with no gasjet, just some Ultra Copper from Permatex.  The other head was obviously eroded, so I used the gasket supplied by Cinnabar (Fel-Pro, IIRC). Both sides were carefully cleaned, broken bolts drilled out and retapped (aaargh!!!!), and tightened with a torque wrench.  Neither was retorqued.
  The test was last summer when we did a 5,000- mile run to New England and back. (We did not spare the horses on that trip.)   A couple of local trips, and I rechecked things this summer.  The side with the Fel-Pro (a respected name in gaskets BTW) was intact.  The side sans gasket was leaking badly.
   I stilll think one should do as the factory did and use no gaskets, but ONLY if all surfaces are PERFECT.  Myself, I pulled the leaking left manifold, cleaned everything up, and reinstalled it with Mr. Gasket's solid copper gasket.  Only a few hundred miles, but so far so goood.  I'll let y'all know how it holds up.Rick
I recently had to remove my left exhaust manifold. I got lots of information and advise from GMCnet and from the GMC Motorhome International Newsletters. It was pointed out that the center bolt is the one that will most likely break. This is because the other four have a hole that exits at the spark plug indent and can be lubed with penetrating oil. (Some people have drilled a similar hole into the center bolt hole.) Put the oil in and give it time to penetrate, then heat the bolts and add some more oil. This should work. I was lucky, all bolts came out without breakage. Using the advise from the newsletter, I had the manifold resurfaced, cleaned the mating surface on the head with a wire brush and emery cloth and reinstalled the manifold using "Permatex" Ultra copper high temp gasket maker. I also used new grade 8 bolts with the old thick washers. So far it is working well. The copper gasket maker is suppose to transfer heat better that gaskets. good luck. Skip
If you break one head bolt off at the block, first weld a small a washer to the bolt then weld a nut to the washer and bolt. The washer is for more area for the nut to hold onto. Use a small rod 3/32' 6013. After welding take a wrench and move the welded assmembly back and forth slightly until it starts to come out. The heat from welding will aid in removing the broken off bolt. Be careful that you don't weld the bolt to the block. I suggest you practice on a test piece of iron first. It'll take a little practice but will have results.Bob
Left 403 exhaust manifold will fit a 455, right will not.tom

Toronado Manifolds
From my experience so far the toronado manifolds are NOT the same as on the coach.  They not only are at a different angle to the cylinders (not a problem) but do not have reinforcing ribs along their length. the ribs help to stop the warping of the exhaust manifold.

I used one from a Toronado for years, until I got headers. It fits the
engine fine,---New ones for the GMC last I knew were about $150 each and available.

 800-544-4761  and  Fax 707-544-4784

Just yesterday I had Flowmaster #52558's and 3" exhaust installed on my 455 so the sound of old and new are fresh in my mind. The new ones are definitely louder (deep sound) when you plant your foot in it but only a little louder when cruising at steady hwy. speeds. (I already had Thorley hedders, installed by prev. owner.)

Incidentally, the conversion made my engine run leaner. Here are emission
test results before and after:

              before       after
CO% - idle     2.0%        1.1%
CO% - 2500RPM  3.1%        2.0%

I am going to turn the idle adj. screws 1/8 turn richer.Richard

403 VS 455

HOW TO ID A 403 VS A 455

Here is some good information on both engines

What kind of mod's would I be looking at to replace the 455 for my 403.  Like  will my Hursh headers hook up, tranny, motor mounts etc?  Or is it all worth  the effort, or just stick with 403 until it gives up, and runs out of steam.   Just don't what that to happen in the high Rockies boonies in Alaska.

The 455 will virtually bolt in there as far as mounts, trans, and headers are concerned. You will need the Toronado intake manifold or else you have to raise the floor. You will need the brackets for the alternator, power steering pump, and a/c compressor from the 455. If there are 4 brackets from
the engine to the fan shroud on your coach, the 455 uses different ones. I know the 455 Toronado brackets are the same but don't know if rear wheel drive cars had the same ones.Denny

I have 78K miles on my 403 engine and switched to Mobil 1 15-50 wgt. oil and went from using a qt every 500 to 600 miles to 1 qt every 1500 to 1800 miles. If I drive at 65 to70 mph  I will average 1 qt every 1500 miles. But most of my driving is done at 55 mph and I average 1 qt. every 1800 miles.

One upgrade that really "perked  up" the performance of my 78 was to install the 3.55 final drive.  Where do you live?  If you tow and live in the hills or mountains the final drive change is well worth it!
If you decide to swap engines I would be interested in purchasing you 403 for a back-up, but I believe you have the most dependable engine for your Tony

Unless the 455 is from a Toronado, you'll need the oil pan, front motor mount, headers, water pump and intake from a Toro or 455 GMC to complete the swap.  The 455 will bolt up to your existing TH 425 transmission with no modifications. You'll also find out that your 403 carburetor will not be sufficient.  A holly 750 cfm with vacuum secondaries has been used by some with success.  I am unsure of the air cleaner fitment but I bet it will work.Steve

1   The block deck is higher on the 455, but it's A LOT higher, about an inch!  The 455 actually has a SMALLER bore than the 403, and a MUCH longer stroke.  (455 is 4.126" bore X 4.250" stroke, 403 is 4.351" X 3.385")

2  The above provides one positive id if the head(s) are off:  the 455 has the 4 1/8" bore, the 403 has the 4 3/8"+ bore.  The Olds FAQs provide a couple of other easy distinguishing marks.  One, the 455 block is so much larger that the heads are 14" apart measured across the top of the block/bottom of the intake manifold, whereas the 403 is only 12" between heads.  The other easy id involves examining the sides of the block.  According to the Olds experts, the 403 should have its displacement cast into the block just above the center core plug.  No such mark on the 455.

 I recommend anyone interested in the details of our engines check it out at:    http://www.442.com/oldsfaq/oldsfaq.htm

455: 212 HP @ 3400, 344 ft-lbs @ 2400, CR 8.5:1 91 Octane or higher
403: 200 HP @ 3600, 330 ft-lbs @ 2400, CR 7.9:1 87 Octane or higher

 In our immediate case, the "Average" octane still mandated today is usually about 5 +-  points lower than the same gasoline would have been rated under the old "Research" method.  Bottom line, you can probably use the 87 octane fuel with no problem.  I use it in mine.Rick
1978 GMC motorhomes came with the 403 engine, not the 455.  The switch
was made in late production 1977 model year.pete
The 455 can be easily identified by the number/letter on the head.  The 455 will have a letter on the head at the front corner on the drivers side. It may be on the back of the passenger side as well I have never looked there. The letter may be A-K but will most likely be a J.  The 403 will have a number, maybe followed by a letter.  It should be 4A.Mumert

"I need to replace my enginge and transmission in my 1975 Glenbrook....I can get a 403 engine out of a 1978, with only 28,000 miles on it for a very reasonable price. Has anyone else made this engine change and what is the general consensus about this change."

Charles -- "Go jump on that 403" - I have owned and driven both engine types - For me, I like the the 403 (small block) vs the 455 large block. The 403 produces 190hp & 315 torque vs the 455s 212hp & 344 torque - not a great deal of difference - but enough that I would get 1-2 miles per gallon better with my 403. Yes the 403 is probaly not as strong as the  455 but they are BOTH bulletproof engines. I prefer a 3.55 final drive with the 403 & I tow a Toyota tercel all the time (10 mpg in flat land Florida).

I believe the 403 is 50 to 100 lbs. ligther than the 455 & because it is about 2" narrower than the 455,a bit easier to work on. Pete


Engine Rebuilds

Cost information on Bdub's FAQ site.


Olds 500 FAQ site, by LarryW


Joe Mondello
How to rebuild a 455 by Mondello.  Good articles.

Article from American Hot Rod

A fellow by the name of Jim Statkus, in Albuq, will do  for about $2850 if you send him the motor.  Call him at 505-765-1614.   Would not be my choice, he has somewhat of a tail light warranty.  gene

I will give you what Dick Patterson uses in his motors for parts.

Pistons:  Silvolite Cast Piston (Hyperuteic) 8.25 comp ratio
Rings: Molly filled rings
Cloyes timing chain
Cam: overlap 206/214 @.050
Some examples:
Wolverine blue racer WG1112K
Mondello JM14100
Crower 56260
Oil pump: High volume Melling    Jeg's part # 689-M22FHV  around $40
oil pickup should be 3/8" to 1/2" off bottom of pan.
All machine work on the cyclinders should be done with a deck plate and the main bearing torqued including honing. Square the block deck with the crank
If they have to replace the seats in the heads make sure that they use Olds seats (5/16") because the Chevy style will not fit correctly and destroy the heads.
Balancing is good
Alway use a new harmonic balancer for rebuild
Magnaflux the crank and rods and have the rod bolts replaced with ARP rod bolts when reconditioning the rods.
All tolerances should be on the light side.
If you have any engine rebuild question you can reach Dick Patterson @705-325-4554
J.R. Wright

 I said I'd post a cost estimate on a 455 rebuild and I've finally gotten it together.  These are fairly universal prices and expect to pay similar from a quality shop/supplier.
Pistons (8.5:1 Badger)                                          320.00
Rings                                                                     57.00
Cam & lifters (Crower)                                         150.00
Rocker assy/chrome moly pushrods (Crane)         197.00
Oil pump (Melling)                                                 40.00
Bearings (rod, main, cam)                                    127.00
Timing chain set (Cloyes t-roller)                          107.00
Gasket set                                                             40.00
Hard seats (Ex. only)                                             55.00
Water Pump (S.D. Pump exchange)                      65.00
Intake manifold & Gaskets (Edelbrock)              360.00
Core plugs                                                            6.00
Paint, sealers etc                                                  18.00

Hot tank & mag block                                         80.00
Bore & Hone                                                    121.00
R & R Pistons                                                    53.00
Resize rods                                                        68.00
R & R cam bearings                                           27.00
Cross Drill & regrind crank                               206.00
Install hard seats/3 angle valve regrind               383.00
Balance                                                            140.00

Sor for a grand total (minus assembly labor) you can have a new engine for approximately $2,800.00
if you do the assembly yourself.  I allow 12 hours for assembly as I'm a little slower than a production shop.  Don't forget all the running around, picking up parts, getting that little plug for the rear oil gallery etc.  It all takes time.  For the final results, click here:
HTH someone,Steve F.


Talked to the regional representative of Jasper engines today for over 2 hours in person, about engines for the GMC motorhome. They are back in production for the olds 455, in case everyone has not been told.

we even called the factory just to make sure and they are ready now. They come with a 18 month/75,000 mile Nation wide warranty.

The 455 comes with a hardened crankshaft, class 1 cam (RV cam), completely balanced, new pistons and rods, hardened exhaust seats, $165 core charge, and total cost of $3797. I can get about a $300 discount off that if we order thru my shop here, delivered anywhere in the US.
They also told me they will custom build one for us to any specs for 10% over that price. We tell them what we want and they will build it. They will also build one for you and keep it with your name on it in their warehouse ready to ship if you break down somewhere.

The toll free number is 1-800-827-7455 or email sales@jasperengines.com

>Quote_for = Engine - Long Block
>Year = 1976
>Make = GMC
>Model = motorhome
>Engine_Size = olds 455 CI (toronado engine)
>AT = on
>AC = on
>PS = on

I am currently having mine done and they took it out the bottom.  The shop is doing it complete with heads and a list of the "good" stuff, - RV cam with entire new kit of lifters spings etc., Stainless valves, forged pistons 30 over, turned crank, new ARP bolts on rods & crank, hi vol oil pump, all new Cleavite bearings etc. and the cost will be about $3,500 turn key.  It took them about 4-5 hours to pull the engine.

He also quoted me 900-1100 labor plus 200+ to do the heads and install a short block from Jasper.  And they run about $2,500 plus shipping.marcus

He described in detail what separates his 455 engines from everyone else's, and why he is not interested in competing with Jasper or Ninex engines. There is allot of hand work that goes into his as well as top quality parts. And yes he makes several engines specifically for the GMC motor home.  He sells 8-10 a month now.  He flatly states that any of his engines can be run hard in the GMC motorhome and will absolutely not fail. He builds them to be tough! No spun bearings in this engine!  He is going to send me a lot of technical info on the 4 engines he makes for the GMC as well as all of his technical literature, and all of the technical articles he has wrote. Thomas

Click for Detail Just a little info: In the $7500 engine he uses the original block, crank and heads (ported and polished) but does extensive work on them  He does a lot of work on the oil galleries of the block, oil restrictors to keep the oil in the crank, replaces all plugs, line bores the crank bearings, all ARP fasteners, High Volume oil pump, special chrome moly oil pickup tube, special oil pump baffle, windage tray, special mondello oil pan with one quart more capacity than the original, special metal and gasket oil pan gasket.  Guaranteed not to leak.  all neoprene gaskets, cross drilled, shot peened and chamfered crank.  Polishes the inside of the stock manifold for better flow, special roller rocker arms, bronze valve guides, stainless steel valves, forged pistons, chrome moly rods, special piston rings hand fitted, special cam with cam buttom (ball bearing end) to get perfect ignition timing, and indium coated bearings (same as Mercedes Benz).  Wow! First class from everything we discussed. This engine is for the guy that positively does not want to break down on the road. He says these engines are bullet proof. He says he has never had to replace one yet since he has been building them.   Thomas

Joe Mondello quote for heads $1399, $1050 to install.-- see web page for all goodies. gene

HEI Ignition

GMC Ignition Systems by Duane Simmons
Many of us old time GMCers (10 years myself) have found that HEI Ignition failures are primarily cause by improper spark plug gaping.  The OEM specified gap is 0.080" which is to great a gap & causes excessive Coil voltage, which can result in all kind of Ignition damage. The solution, w/o sacrifice of performance, is gap plugs at 0.060" ( AC R46SZ. Also use only Delco Ignition Modules & apply an abundance of the Thermal Conduction Grease on the base of the module during installation. Recommended spare Ignition parts are as follows:
 1)  Delco Ignition Module w/thermal grease
 2)  Ignition Coil  (Delco only)
 3)  Rotor (Delco only)
 4)  Distributor Cap (Delco only)
 5)  Spare Ignition wires (2-3 wires)

Also failures occur due to lose of the Coil ground which give erratic or
non operation conditions (check for looseness on the Coil mounting screws
& place both ground wires under one mounting screw).

The GMC Motor Home Distributor is unique in its mechanical & vacuum
advance curves & therefore a car dist. will not provide performance w/o
modifications.  Try Bob Lamey, 1-909-982-7747 Ontario, CA for an expert
distributors Upgrade (new GMC Motorhome Distributors are not available).
I too do not think a failure of the module is justification for
replacement of the distributor (if not done properly, a step in the wrong
I work at Dana Corp who recently bought Echlin who supplies many NAPA parts. Several years ago I was the project engineer for ACCEL distributors made here in Branford Ct.
as part of my job I had occasion to have ignition modules tested with a variety of different ignition coils.  We actually documented the fact that some TP-45 type modules would not fire some coils at a given range of RPM (generally the ones made off shore). The module would literally drop out and not fire the coil for instance from 1,200 rpm to 1,250 rpm (this can cause really hard to diagnose performance problems). I am not an electrical engineer I just know what we recorded. Bottom line, it is best to get a coil and module for the same application and probably best from a dealer. I wish I could say that I am sure the NAPA TP-45 modules are the good ones, I can't, but I bet the $10.99 unit is made off shore.
rebuild is about $190 and has new shaft, lower bearing and guts, Bob does them for Cinnabar--
shipping addr is    321 Armsley Sq.
                              Ontario , CA 91762-1606
Don't throw away or replace that old HEI unit.  They're rebuildable. GM P/N 1894379.  This is for a replacement shaft kit that includes bushings.  They're about the only parts of the unit that can wear out. If you're worried about the advance curve, send the whole thing to GES engineering here in San Diego. He will professionally recurve yours, or your replacement, and install an adjustable vacuum advance kit. Runs around $100 plus parts.  He'll also provide the curve specs.  He also rebuilds/recurves older points type units. Steve Ferguson
An HEI tester is available at many Auto Parts stores for about $10.  A
must for simple HEI trouble shooting.  The HEI Tester is made by K-D
Tools p/n 2756,  "Calibrated Ignition Tester for HEI Systems".

Hot To Test a HEI distributor(9/17/06)

Engine Cranks But Will Not Start

Turn the ignition off, remove the air cleaner, and look down the front carburetor barrels. Depress the accelerator once or twice while looking for gasoline squirting into the front barrels.
Gasoline present: go to Step 1
Gasoline not present: Fix missing gasoline problem.

1. With the ignition on, connect a voltmeter (or test light) to the + post or HEI BAT terminal located on the distributor cap.
+12 volts or Test LIGHT ON: Go to step 3.
No voltage or Test LIGHT OFF: Go to step 2.

2. Check for continuity or voltage drop in the ignition switch and in the circuit from the switch to HEI BAT terminal. Fix missing +12 volt problem.
STARTS: Stop—you’ve found the problem.
NO START: Go to step 3.

3. Remove a spark plug wire and insert a HEI spark tester ($2.99 from Harbor Freight) in line. Crank the engine and observe the light in the tester.
3a. Alternatively remove a spark plug wire and insert a metallic extension in the end of the wire. Using insulated pliers, hold the spark plug wire no more than .25 inch away from the engine block while cranking the engine. If you use the alternate method be very careful to hold the wire no more than .25 inch away from the engine. Greater distances can cause damage to the ignition module.

LIGHT FLASH IN TESTER or SPARK JUMP TO ENGINE BLOCK: The problem is not the ignition system. Check the fuel system, spark plugs, timing, and internal engine condition.
No light or NO SPARK: Go to step 4.

4. Remove and inspect the distributor cap for cracks and carbon-tracking. Visually inspect the distributor mechanism for moisture, corrosion, or burns. Repair as needed.
STARTS: Stop—you’ve fixed it.
NO START: Go to step 5.

5. Set an ohmmeter to the low scale. Connect the leads across the HEI’s “BAT” and “TACH” terminals.

ABOVE 1 OHM: Replace coil. If vehicle still doesn’t start, go to step 6.
0-1 OHM: Go to step 6.

6. Connect one ohmmeter lead to the “BAT” terminal and the other lead to the coil contact in the inside-center of the cap. Switch the first lead to the “TACH” terminal and recheck.
EITHER TEST READS 6K to 30K: Coil is OK. Go to Step 11.
BOTH TESTS READ INFINITY: Replace the coil. If the engine still won’t start, go to step 7.

7. Remove the green and white leads from the HEI control module. Connect the ohmmeter from ground to either lead.
INFINITE reading: Go to step 8.
LESS THAN INFINITY: Replace the pickup coil. If the engine still won’t start, replace the module.

8. Connect the ohmmeter across the green and white pick up coil leads.
READS 500-1,500 OHMS: Repeat Steps 7 and 8 while moving the vacuum advance with a screwdriver. If reading is still OK, replace the module.
DOES NOT READ 500-1,500 OHMS: Replace pickup coil. If the engine still doesn’t start, replace the module.

That is it. You will notice that there is really no diagnosis of the module. When everything else reads correct the module is replaced by default. In my case the coil read open. replacing it did not fix the problem so I had to replace the module also. BOTH items were bad and I assume the coil took out the module. HTH Ken B.

Upgrading to a HEI Ignition
I think you will find that the GM HEI ignition is very hard to beat for the Motorhome application.  You will find many GM cars that have an HEI distributor that will fit the 455 & 403 engine.  However, the vacuum & mechanical advance will need to be modified to need our needs.  The coach will run with these car distributors but not very good & damage can be done to the engine.  Best to buy from wrecking yard a GM HEI distributor & have it upgraded to the correct curves. Look for one with a GM only Electronic Module, rotor & cap (other after market brands are not as reliable).  Bob Lamey (909-982-7747.....Ontario,  Calif)) is an excellent source for this upgrade.  Bob has a distributor machine that he verifies/adjust both the vacuum & mechanical advances to our needs.  He only needs the lower part of the distributor (not the coil/Cap). The change over requires the following:
 1  Off set Air intake filter housing (from HEI car...olds or other)
 2  New power wire from Ignition to Dist. Input
 3  New Spark Plugs:  AC  46RSZ  (0.060")  not 0.080"
 4  New Spark Plug wires (HEI type with metallic conductor)
 5  Set Initial Timing at 8 deg. BTDC at Idle

Note: Most, if not all,  car dist housing has a notch to index the Cap at 180 deg. from where it should be for the Motorhome application.  File new notch for installing Dist Cap with input connector near the Vacuum advance Unit (180 deg from original if required).  Don't want the input connector on port side near the accelerator linkage.

This Upgrade was one of the best improvements that I did to my coach (about 9 years ago).  Well worth the effort. Duane Simmons 

Cable harness for the conversion


Great stuff and a real nice guy! I order an HEI-1 and an ALT-1 from him.Terry Taylor

Page 6Y-45 and 6Y-59in the X-7525A factory service manual (FSM) specifies (the distributor turns)

Page 6Y-33 in the X-7725 FSM specifies clockwise, which is wrong.

Dick Paterson always points out in his GMCMI convention seminars that Olds is
the opposite of other vehicles and the distributor spins counterclockwise.
   Paul Bartz


I contacted Dick Paterson and he gave me permission to publish the following info:

Distributor Specifications, Type: HEI - Modified
Application: GMC Motorhome
Engine: 455 Oldsmobile
Final drive: 3.07
Tow: No

Initial setting: 12°

RPM   Advance
800     2°
1000    5°
1250   10°
1500   12°
1750   13°
2000   14°
2250   16°
2500   18°      30° = Total

6"     0°
8"     2°
10"    6°
12"   10°       40° = Intial + Centrifugal + Vacuum

As I understand it he has developed a number of different curves for GMC's with different final drives and whether they tow or not. ROB

I used the HEI distributor from a 75 Olds Toronado (Cardone #1893) and changed the vacuum advance to 10 degrees (Delco #1973577). You also need to change the wires and plugs. Use .060 gap plugs, (not the .080 ones). The larger gap plugs tend to burn holes in the rotor. I use AC/Delco R44SX plugs but they are getting hard to find. When you install the distributor you must run a heavy wire (12 ga or better) directly from the distributor to the ignition switch. This new wire will bypass the resistance wire that powers the old ignition system.

You will probably have to modify the air cleaner so it will clear the larger distributor cap. I used a hammer and it worked find. Or you can get one from a junkyard 75 olds Toronado.

It would probably be a good idea to check the timing curves after installing the new distributor. Here is what I have been told they should be for the GMC:

       Speed   No Vac   With Full Vac
       Idle         8                   18
       2000     18
       2500     20
       3100     27

In order to check the timing more easily I painted the harmonic balancer black. Then I painted white lines at 8, 18 and 27 degrees BTDC. I calculated the positions of these marks by measuring the circumference of the harmonic balancer (20.5") and dividing by 360. This gives the length, in inches, of one degree on the surface of the harmonic balancer. The zero degree mark (TDC) is the reference line notched into the harmonic balancer. When I cranked the engine so the zero degree mark is on the drivers side (looking up from under the engine), the BTDC marks are toward passenger side in order as follows:

The 8 degree mark is at .46". This is the timing at idle with the vacuum advance disconnected.

The 18 degree mark is at 1.03". This is the timing with the engine at 2000 rpm and the vacuum advance disconnected. It is also the timing at idle with full vacuum advance.

The 27 degree mark is at 1.54". This is the timing with the engine at 3100 rpm and the vacuum advance disconnected.

I used white paint to make the lines marking each of these timing positions on the harmonic balancer.

I disconnected the vacuum advance from the carburetor. Using a timing light, I set the timing (at idle) to 8 degrees. If the lines on the harmonic balancer are painted correctly the 0 degree mark (the notch in the harmonic balancer) will line up with the 8 degree arrow on the engine and the 8 degree mark (the first painted line on the harmonic balancer) will line up with the 0 degree arrow on the engine. Note that the zero degree arrow is the first point on the drivers side. The other arrows on the engine are spaced 4 degrees apart (4, 8 and 12 degrees).

To check the centrifugal advance, I ran the engine up to 2000 rpms and checked the timing. If the centrifugal advance is working properly the 18 degree mark on the damper (the second painted line) will be lined up with the zero degree arrow on the engine. Then I ran the engine up to 3100 rpms and checked the timing. If the centrifugal advance is working properly the 27 degree mark on the damper (the third painted line) will line up with the zero degree arrow on the engine.

The centrifugal advance on my distributor is pretty close. At 2000 rpms it was right on at 18 degrees. At 3100 rpms it was at 24 degrees. Thats not quite as advanced as I have been told it should be (27 degrees at 3100 rpm) but I rarely run much above 2500 rpms so I live with it.

Before checking the operation of the vacuum advance be sure that the idle timing is set to 8 degrees when the advance hose is disconnected. Then unhook the hose that goes to the transmission modulator and connect the vacuum advance hose to the transmission modulator port on the intake manifold. This will apply full vacuum to the vacuum advance and should pull in an additional 10 degrees of advance. Use the timing light to verify that the 18 degree mark painted on the damper lines up with the 0 degree arrow on the engine (8 degrees idle plus 10 degrees vacuum advance).

If you see more than 18 degrees then you have the wrong advance and should replace it with the correct one - Delco 1973577. If you see less than 18 degrees then the advance is defective or there is a vacuum leak or blocked hose.

The curve on this distributor (75 Toro) is pretty close (after I replaced the vacuum advance). Here is a summary of what I measured:

       Speed   No Vac  With Full Vac
       Idle         8                   18
       2000     18
       2500     20
       3100     24

There are a number of places that will rebuild and properly recurve the distributor for you if you would like it to be exact. Bob Lamay and Duane Simmons are two people that where recommended to me.Dave


The guy who set up my timing over a year ago said not to worry about the base timing , vacuum advance, and flyweight advance, just set the total advance to 35 degrees and go from there.   How wrong he was.

click for detail He did not check the vacuum advance that he installed with the engine, and come to find out it was adjustable and was set for 25 degrees of advance.  So when that total advance was set for 35 degrees, minus the 10 degrees of the flyweights, the base advance was ZERO (0)....  As a result I always had this flat spot in the acceleration when the vacuum dropped at startup , the timing dropped down to the base advance of Zero and the engine would act like it hit a wall until the vacuum would again advance the timing.  The picture shown above shows the setting I found when I started to time the engine, and the top arrow shows where I set the final position yielding 10 degrees of advance.  The engine now accelerates smoothly through the range and I have overcome a major problem.  Thanks to Dave's great article above on how to check the distributor curve.   My numbers are now:

idle        7        18
2000    15
total     30


Spark Plugs

Here is a link to JR's spark plug table (11.24.10)



AC Delco Rapid-fire #7 are a platinum electrode plug that is rated
for 100K mileage.  Recommended by Dick Patterson and others.  Run
them as they come out of the box.   It is about a 38  to 40 gap from
the factory.  Much easier on the HEI.  JMHO J.R. Wright

I use Bosch single plat plugs  # 4023, put then in a few years ago and still running good, this is the plug we use here at MGM unless the customer want another plug. @ 2.99, can't go wrong -

The Rochester Files

Quadrajet stock calibration
OK, here is what I know about the Quadrajet stock calibration.  This information is a composite of a 1991 Caughlin Coach Talk, Doug Roe’s book and the big Rochester parts book.

Year    Carb No.   Primary Jet   Primary Rod     Secondary Rod      Sec. Rod Hanger
73-74   7043254   70                 50D                  CJ                          V
Fed      7045254    70               50D                  CJ                         V
CA       7045554    68 `              50D                  CJ                         V
403     17057254    67  7043541                       CK                        V
 78      17057559(CA)
           17058254    67  7043541                        CV                         V

The Carburetor Numbers are on the left side, in front of the boss that runs vertical beside the secondaries.  Probably unreadable with the carb on the vehicle unless you use a magnifying mirror.  There are other subtle differences, such as the power spring (4-8 in hg) hot unloader (not present) for the 455 and secondary air valve setting that are unique to the motorhome.
It is important to make sure you have no vacuum leaks and that you have the correct line connections.  Both problems that I have seen and experienced.  The addition of a vacuum gage and an air/ fuel ratio meter are helpful in keeping track of your carb performance.  An air/fuel
meter is available from Summit, Edlebrock, Westach and probably others, or you can make one from a three wire O2 sensor and a volt meter.  1 volt = 14.7 A/F. As Arch says, This is what I know (and maybe a little more). I did give a seminar at a Western States rally on the subject once.  Would appreciate comments corrections and additions from others. ps I'm a digest subscriber so it may take a day for responses.
Frank Condos
76 Glenbrook
Monrovia, Ca

Most alternators have been changed on the GMC motor homes.  It seems at that time a little spacer under the mounting brackets that go to the side of the engine block, get lost.  If this spacer is  not there, the pulley of the alternator will not be straight with other pulleys and the belt will wear out fast and often jump out of the pulley.  Here are some pictures that show the OEM mounting brackets of the 455 and where this spacer is located:

Here is another way to fix the alignment problem

Why does the alternator light stay dimly lit?

As a test, the Alternator light SHOULD BE ON when the key is on and the engine is stopped.  If it is not, replace the bulb.
The Alternator light SHOULD NEVER BE ON, dim or bright, when the engine is running.
If it is, you should do the following:
Install the APC cable.
This will  eliminate erroneous indications that will keep you from finding the problem.
If the light is still on after installing the APC, the battery voltage is higher than the alternator voltage.  Use your voltmeter to find if the alternator is not working or something else  will not allow  the alternator voltage to reach the battery.  The link above describes several things that could cause this, like a loose belt.
The alternator light is a very valuable diagnostic tool and an important part of the charging circuit.  It should never be removed or ignored.   Always look to see that it comes on when you turn on the key and the engine is stopped.
The OEM charging circuit is not faulty.  The Nichrome wire will not get hot with the key on.  I have run tests with the actual nichrome wire that show the wire will not get hot at even 40 volts.


This is ususally  a normal condition on pre 77 / 78 models of GMCs".  The easy way to solve this problem is to order the Alternator Protection Cable (APC). The APC will eliminate the Dim Alternator Light , and damage to the Nichrome Wire. If you look at the pictures in the following articles, you will see a lot of wire tracing, crimping and soldering.  The APC provides the protection with out all of these complicated modifications. 

To order the APC Assembly go here:


Not everyone knows the functions of this light.  If you have the original GMC alternator and are using the factory style indicator light on your dashboard, it is a pretty helpful thing. It helps kick-start the alternator into working at idle speeds when you first start the car, and it tells you if the alternator is putting out less voltage than the battery has in it, indicating a problem.

The light is connected on one side to the field current system inside the alternator and to a switched ignition power source on the other side (to the fuse panel inside the glove box.) When you turn the key on but have not started the car yet, the field acts as a ground and power flows through the light and out to ground - lighting the bulb so you know it works.

Once you start the car, the voltage at the field is powered internally by the output of the alternator. If this value is exactly the same as the battery voltage, then you have the exact same voltage on each side of the indicator light and they balance each other out so the light doesn't light. If all goes well, the light never comes on, and you drive happily around knowing all is well with your alternator.

If the output of the alternator should drop due to a slipping/broken belt or due to certain kinds of electrical faults inside the alternator itself, there will be less voltage on the field side of the light and more voltage on the switched ignition side of the light. The result is that some amount of electricity will flow through the light and into the field and the light will glow proportional to that voltage difference. This is how a slipping belt or an overloaded alternator will cause the light to glow very dimly, while a full-on failure will cause the light to glow very brightly. Note that if you disconnect (or forget to connect) the wire at the alternator, the light will never come on and the alternator may not charge properly.

GMC Tech Bulletin
"Technical Service has received several reports of the generator telltale light glowing dimly at all times.  This condition can exist even though the alternator is operating correctly.  It may be caused by a partially discharged vehicle battery or by the small voltage drop across the dual diode isolator.

If the above condition is encountered and the owner of the vehicle expresses concern over it, the following procedure should be followed to assure that it is not being caused by a malfunctioning alternator.  The owner should then be assured that the condition is normal and will not cause any trouble in the system.

1.  Disconnect the battery ground cable.
2. Connect an ammeter in the circuit at the "BAT" terminal of the alternator.
3.  Reconnect the battery ground cable.
4.  Turn on radio, lights high beam, and blower motor high speed.  Connect a carbon pile across the         battery.  {Emery's note: this is to put more load on the battery.  Try turning on a lot of 12 volt appliances in the GMC if you don't have a carbon pile}.
5.  Operate the engine at a moderate speed as required, and adjust the carbon pile as required to obtain a maximum current output.
6.  If ampere output is within 10% of rated output (80 amps) alternator is not defective."  (Emery Stora)

Cut the brown wire going to the idiot lamp.  Insert a 25 cent Radio Shack diode in the line in any direction.  Turn on the ignition to test.  If the light comes on, you're done, just start the coach and watch the light .go out  If the lamp does not come on with the ignition, reverse the diode (end for end) and you're done.  Get a Radio Shack part number 276-1102 or similar.  (John Dolan)

Plug in an APC cable to stop this problem


 There is one other condition that can cause the symptom of a dimly lit idiot lamp.   A GMCer replaced his alternator, and still had the dim glow problem.  He checked a diode that is supposed to eliminate  this on some production models. NO luck.  He then discovered that his 10 AMP dash instrument gauge fuse was blown.  The fuse provides 12 volts to one side of that lamp and if it is blown it does not have 12 volts there.  When the alternator starts putting out 15 volts there is a differential 15 - 0 and it gets an indication.  Replace the fuse (or check wiring and grounds to make sure you have 11.5 to 12 volts at the lamp and the glow should cease, except when the alternator is not putting out 15 volts, which is the way it is supposed to work.  It was reported in GMCMN in March 98 page 5.  (John Dolan)


The dashboard indicator light circuit also  has a resistance wire with a calibrated resistance in it. This wire runs in parallel with  the indicator light and has about a 10 ohm resistance. It's purpose is to allow slightly more current to flow to the alternator field current system at initial start-up to make sure the alternator begins producing power as soon as the engine starts. About 1 amp total current is flowing to the field current between the light and the resistance wire, with the resistance wire supplying about 3/4 of an amp. This extra resistance wire does not affect the functionality of the indicator light in any way.

This resistance wire has been known to overheat (perhaps due to bad diodes or a bad voltage regulator in the alternator).  When it overheats it melts the covering of the wires in the wiring bundle that runs under the dash padding by the windshield on the passenger side.  This will short out all kinds of things and could even start a fire.

Someone has proposed replacing this resistance wire with a regular wire in series with a 10 ohm resistor, keeping the resistor away from other parts so that if it overheats it will not melt anything or cause a fire. Emery
The Ni-Chrome wire is an excellent high power resistor and GM made a good choice.  The problem is, when something goes wrong with the isolator, or alternator, a higher voltage is placed across the wire and it gets hot and burns up the wiring bundle and causes a big problem.  These pictures show how much damage is done by this condition.

> in the GMC ckt, the only way to get high voltage is for the regulator to fail in a specific failure mode, this delivers 100 volts , blows up batteries, and burns wires....etc.
see this example
 I was not so lucky and  not only did the isolator burn out, but I  also had the alternator burn up just prior to the battery exploding. Fortunately no one was hurt and this all occurred in a shop where the
AC was being re-chaged and being tested.  Yes, the AC fan was on and that also burned up.  The shop mechanic said he measured the alternator voltage just prior to the explosion at 90 volts and
couldn't shut it down fast enough.  We are not sure what caused the chain reaction but it must have been a failed voltage regulator and probably a bad battery ... but definitely a failed isolator.  Yes, I
now have a combiner and after a $1,200 bill,  everything is working nicely.Dave Bockman 11/07



The only time the wire should draw power is when the alternator is stopped and the key is on.  Then the wire could  dissipate  (12.5 * 12.5) / 10 or 15.6 watts.  Good design says if you replace this with a discrete resistor it should be 2 * 12.5 or a 25 watt resistor.  this is a large resistor and hard to place, so the Ni-Chrome wire is a good choice.   Also, the diode should be placed between the alternator and the resistor and the alternator light like it is in the APC.

Some shops have replaced the Nichrome wire with a 100 ohm resistor.   This is a modification of the GM design and  sometimes creates  a sluggish startup of the alternator.
See these examples:
> I tried an experiment a few years back I disconnected the nichrome wire circuit to the alternator and ran that way for sevral months.  My alternator always started.  When Lawrence tried the same thing his would never start.  So we definitely need some kind of start circuit.  >
100 ohms provides  .12 amps will work though I did see one coach  modified that had to be reved up to about 2000 RPM to get it to start charging. kenB>
There are other famous examples of  the 100 ohm resistor problem
( emails from 22/21/07)
 removed the hot wire and installed a stranded wire and a resistor. If my coach has been sitting for a long time it will show almost no voltage for 2 or 3 min. Then all of a sudden it come up. Take Care
The reason your alternator is slow to come up is that resistor  I'll lay dollars to donuts it's 100 Ohms, ten times the OEM 10 Ohms.  Reduce  it's value back closer to the 10 Ohms and your alternator will receive proper initial excitation of its field. Ken H.>

********* see pictures to view the permanent wiring changes.

Pre 77/78 models  http://www.gmcmhphotos.com/photos/showgallery.php?cat=3247

     77/78  models  http://www.gmcmhphotos.com/photos/showgallery.php?cat=3254

 damage done        http://www.gmcmhphotos.com/photos/showphoto.php?photo=1546&cat=3139


1.  Disconnect the shore power to kill the converter.
2.  Disconnect all batteries.
3.  Remove the glove box to gain access to the fuse panel.
4.  Remove the screws holding the fuse panel to the fire wall and unplug the connector on the outside of the firewall from the fuse panel.
5.  Locate the brown with white stripe wire (if you roll that wire between inside) and extract the connector from the back of the fuse panel.  On the outside of the firewall, this lead connects to the brown 16 gage wire leading to terminal 1 on the alternator.  Its purpose is to provide initial excitation current for the alternator field (since there's very little residual magnetism in the rotating field of an alternator, a little bit of "excitation current" is needed to get things started; as soon as the stator starts producing current, it takes over and this lead is superfluous).
6.  Cut the connector off and solder it to a 12" length of 14-16 gage wire, then re-insert the connector into the fuse block.
7.  Tape the cut end of the resistance wire for short protection until you locate and disconnect the other end near the ignition switch.
8.  Locate the 12 gage brown wire which leads to the accessory buss bar in the fuse panel and attach to it another 12" length of 14-16 gage wire.  I stripped 1/2" of insulation from the 12 g. brown wire and soldered and taped the 12" wire there.
9.  Mount a 10 ohm, 20 watt ceramic resistor (Radio Shack should have one, preferably with an insulated mounting bracket) in a convenient location near the fuse panel.  Attach one of the two added 12" wires (cut to fit) to each
end of the resistor.  Be sure no conductor can contact any exposed 12 VDC terminal.
10.  Put everything back together.
11.  Reconnect the batteries and "smoke check" everything.
12.  Crank the engine and check alternator output (battery terminal voltage) for 13.5 VDC+.
13.  The next time you pull the instrument cowling, refer again to Lionel's picture and try to find the splice where the resistance wire is connected. If you find it, cut it off close and tape the bare ends.  While there should be no current through the wire after you've cut the other end, it is conceivable that it could still be shorted to ground somewhere and cause the infamous "smoking wiring harness".HTH Ken Henderson

Dual Belt Alternator Pulley
I have always had problems with the alternator belts on my 80 Amp alternators. When I replaced the last 80Amp alternator with a 100Amp unit I decided to replace the single belt pulley (that comes with the 100 Amp alternator) with a dual belt pulley. The pulley I found is a bolt-on replacement made of machined steel, 2 5/8" dia., part number 1101, manufactured by BBB Industries. It costs $7.88 and can be ordered from Kevin at Hedahls Auto Parts in Bismark, ND. (800-472-2112). The 2 5/8" diameter pulley is small enough so that the 100 Amp alternator will generate 80+ amps at engine idle.

When I replaced the alternator I also replaced my belts with two Gates XL-7570 (3/8" x 57-5/8") for the alternator & PS pump and one Gates XL-7619 (3/8" x 62-1/2") belt for the A/C compressor.

I only have about 3K miles on the 100 amp alternator with a dual pulley but so far so good. Even with the alternator putting out a full 100Amps my belts stay put and make no noise.

I would suggest replacing all of your belts with new ones. My experience is that when you mix old and new belts, the old and new belts will be slightly different sizes. That causes the older belts to ride deeper in the shiv and the new belts to ride higher on the shiv. Thus the new and old belts fight each other. Since two of my belts share all pulleys, and three belts share pulleys at both the crank and the water pump, I replaced all of my belts with new ones. Dave

Timing Chain Replacement

100,000 mile replacement (Heinz)

In my experiance the single non roller timeing chain stretches at 70-90K miles.  That is not a maybe, it is a for certain.  When that happens, your performance drops off as the loose chain allows the timing to go all over the park.  This condition happens slowly (kinda like how my eyesight has gone bad).  Then you go out and get glasses and then wonder how you didn't no0tice how blind you were!  The timing chain & gears go bad gracefully until that fateful day when it jumps a tooth under load and then the fun begins.

I would say thay the single best thing you can do for your approaching 100K motor is to replace the old (mearly mortal) chain and gear set up with a double roller, true rollerchain/ gear set.  They will never go bad, you can keep them for the next rebuild.  They eill be quiet, reliable and give you the assurance that the crank and cam will be tied to each other no matter what.  Of course while you are in there, I would suggest replacing the water pump, fuel pump, front seal and fan clutch.  All of these components will be off and on the floor while you dig to reach the timing chain.  The labor will be free to replace them and then you will have the added relief of knowing they are hunky dory too.Jim

Installing timing chain without removing radiator and fan shroud. (11/25/06)

While replacing the timing chain I found that the transmission cooler line fittings were rusty and stuck in the radiator cooler.  They were still frozen after using Kroil over several days.  I was afraid of damaging the radiator if I forced the issue too much.  Since I did not want to have to fix something that was not broken, I came up the following work around.  It's a little tight and would probably be easier if  the radiator, fan shroud, and transmission cooler lines were removed.  I am sure others have done some variation of this before but, I thought I would share my technique.  I'm working on a '78 403cid with a full coverage fan shroud.

Installing timing chain without removing radiator and fan shroud. 

1.  Before removing old timing chain, rotate the engine so that the timing mark on the crank gear is down (6:00) and the mark on the cam gear is at 9:00.
  2.  Lay out the new gears with the new chain so that the timing marks are lined up (crank mark 12:00, cam mark 6:00).
  3.  Carefully rotate the gears on the chain so that the crank gear is at 6:00 and the cam gear is at 9:00(&#189; turn on the crank gear and &#188; turn on the cam gear).
 4.  Take liquid paper and mark the chain link that corresponds to the timing marks on the crank & cam gears (I also colored the timing marks on the gears so that they were easy to recognize at an angle and low light.
  5.  As an extra measure use tie-straps and secured the chain to the cam gear and another to the crank gear (Since I was using a double roller set the crank gear has extra key slots to run the tie-strap through.
6.  Install the chain and gears.  The tie straps keep the chain on the gears and the marks provide secondary evidence that the gears are lined up.  Before pushing the crank gear home, cut and remove the tie-strap so that it does not get pinched behind the gear.
John Sharpe

Here is a clear picture of how the marks are supposed to line up out of the GMC


Regards,Rob Mueller

How to Replace the Chain (Removing shroud)

I had a chain I Had picked up from Joe Mondello [Phone (805) 237-8808," & it was a Cloyes Timing chain] last year and now was the time to install. While the Manual is usually good in describing how to do things in this case it is poor and not even accurate ,as it says you have to remove the oil pan and you don't have to, in fact you can't remove the pan!

Every thing can be done from the front-at least in my case as I have already removed the large round metal and rubber spacer between the radiator and the fan (not the fiberglass part attached to the rad.) see Ken Thomas' excellent idea in GMC Motorhome Marketplace April,1997, re getting rid of a lot of the clutter in front of the motor and making this area much easier to access and work in. Having done this ,I've found  this to be  one of the very worth while changes to make to the motor area. This gave enough room to work and get your head up in there to be sure that you get the timing marks in the right place. It is a bit tricky as the new chain goes in with absolutely no slack. I learned from the experience of pressing the chain on and found that I was one cog off, so had to pull it off again.Take your time and eye ball it to be sure you get the marks lined up correctly.

In my case I got some antifreeze into the oil pan so be very careful to drain it out and keep any bits of dirt etc. from getting into the pan when the area is opened at the front. I ended up in draining the oil pan as well as having to do some mopping in there with rags but likely if one got some rags in at the start it would be better and save the extra work and mess. Also when draining antifreeze have back end of coach high and after draining by lowering back end you might save some of the antifreeze from draing foreword from the block.

As I recall I had to undo the front motor mount so you will need to find a way to support the weight of the motor. I used some 2x4's on each side of the motor on the vertical.

I was amazed at how it quieted down the motor,now having a tight chain.
I also find that my Ping Alert is much more accurate as it is not
picking up extraneous other noises in the motor.
Good luck.
Claude in Victoria BC.