The Stovebolt Site Forums The Shop Area Engine Shop Possible Carb troubles

Does anyone out there have any idea whats going on with my 54 GMC? I have a 248 engine, completely rebuilt head, new gas tank, fuel lines, fuel pump and rebuilt carb. Truck ran great for days now it's giving me fits. Starts up idels, runs smooth and strong for 5 mins and then just dies. Then it won't start at all..it's like the key is off,the engine tries to kick over but won't? Anybody...ideas??
Thanks

Does it fire up after the 5 or so minutes of running if you give it a small shot of fuel down the carb?

Have you checked to see if you are getting spark at the plugs? I don't remember if 54 had one but I would look at the balist resistor, if you have one. Just my guess. Good luck and make sure you let us know what it was.

Almost sounds like vapor lock to me. Symptoms are consistent with vapor lock.

When you replumbed the truck, were you careful to follow the original fuel line paths, or did you re-route plumbing in any way? Is the fuel line close to the manifolds in any way or does it pass by any heat sources?

In summary, vapor lock is where the fuel heats and "boils" and then can not flow. Try to isolate the fuel lines and if you have the time and patience, try making up a "jumper" line and install it from the fuel pump up and over the top of the rocker and into the carb so that it is as far away from heat sources as possible. See if this cures the problem. If yes, then you know you need to re-route your plumbing (to the original paths?)

The new formulated unleaded fuels are much more prone to vapor lock than fuels were when your 248 was made. Add to that the high compression of the 248 and you have a formula for vapor lock. This is a problem in aircraft engines too and if you dig around the web, you will probably find quite a bit of information on it.

Also, there are fuel additives that provide some reduction of vapor lock potential, but my guess is that the restoration work fuel line path changed from the original configuration. High test fuels are more prone to vapor lock too. There's an old trick of adding a little diesel fuel to your gasoline to prevent vapor lock, but I strongly discourage it.

Vapor lock is my guess.

Guys...everything you said I found very interesting.....First of all when I add a small amount of gas to the carb,it starts right up and runs smooth.
Warms up as normal to 140 and then stalls out.
Prior to this there were no symptons...it ran great started really quick...I mean it started right up without hardly a crank.
What does the ballast resistor look like and where would it bee on a 54 GMC?
As for the fuel lines, I routed them almost exactly the way they were, I installed a fuel pump with vaccum pump on it for the windshield wipers, purchased from Jim Carters. The only change was the inlet was at a little different location...other than that same route. The outlet does go over the top of the valve cover rather than tightly around the front of it. But it's a good 1 inch higher than the cover.
Patricks rebuilt the head and when it runs the engine is strong and tight.

Old CarKook,
You could probably answer this for me. Do you know the correct downdraft carb. numbers for a 54 GMC with the 248 engine? I actually have (3) carbs for that truck. The one that was on it was a Zenith 11275D, on there now. Next I bought was a Zenith 11280XB, and the third was 10569A it's the worst of the bunch. Have anyway of knowing if these are even the right carb for that truck?
Thanks

You said it ran good [after it quit] when you squirted a little fuel down the carb. If it were me, when it quit, I would remove the fuel line from the carb, squirt a little fuel in the carb. and see if it would run. If it does, see how much fuel is coming out of the fuel line and how much preessure it has. If fuel comes out in a good volume and still won't start then it is in the ign, system. Check to see if it is getting fire to the plugs. It is possible the coil or condenser is no good. Make sure the coil is wired up to the correct polarity. If the truck is positive ground then the wire going to the distributor goes to the positive side of the coil, if it is negative ground, then the wire goes to the negative terminal of the coil.
Good Luck!

My parts books say that a 1954 GMC Model 100-24 calls for Zenith #11618 with manual choke and manual transmission, or any of the following for the HydraMatic in 100-24: #11503, 11578, 11577.

I can not match any of your numbers. :confused:

You could have a bad condensor in the distributor. I have seen them bad right out of the box. After it warms up it breaks down.

Same problem can occur with the coil.(discussed per Email)

Old CarKook, thanks for researching the carbs, your not the first person to tell me that. A couple of carb. rebuild shops have said the same thing. Is it worth trying to find the correct carb? I remember you said you had a carb in your attic parts lot???? Could it be the correct number???
The only thing that puzzels me is that it ran great for a period of time. It drove great.
When you think you have everything figured out and working great they throw you a loop...
I think I'm going to replace the condensor, plug wires and coil. The coil is brand new but a
couple of people have suggested it might be bad the plug wires and coil were on it when I bought
it. Thanks for all of your input all of you!

correction: the condensor is brand the coil and plug wires were on it when I bought it. Replace them all can't hurt!

Sounds like it may be some blockage in the fuel line .
Maybe some lint or rust has been drawn up the line from the tank and is restricting the flow.
Next time it stops , take the top off the carb and see what fuel is in the bowl , check the needle and seat is clear too .

54 if it has a balist resistor which I don't think it does.... It will look like a ceramice block about 3 inches long and 1/2" to 3/4" wide. It will have spade terminals one at each end. It can be mounted to the firewall or sometime mounted to the coil bracket. Good luck tacking down your problem

54 GMC Man - Since the truck will immediately refire when gas is poured into the carburetor, you probably have a fuel delivery system problem.

Items to check:

(A) clogged fuel filter
(B) clogged (or missing) fuel tank vent
(C) vapor lock (I doubt this one if occuring after only 5 minutes)
(D) defective fuel pump
(E) partially clogged tank fuel pickup

11275 and 11280 are both universal replacement carburetors. Check this link for original carb:

http://www.thecarburetorshop.com/TRGMC.htm

Jon.