1968 C2500 engine dying

[gmc396]

I have a '68 GMC 2500 with a 396 engine and granny 4 speed. The last few days have gotten chilly; down to the high 40s. Truck starts right up with one pump, and the choke sets. After I drive a few miles and engine is warming up, when I push in the clutch, the engine dies. I'm able to restart by letting the clutch out. Pulling into a parking space at the end of my trip (about 3-4 miles) it died again. When I came out an hour later, it started fine. This time on the way home, it did not die.

I rebuilt the carb last winter and performed a tune up. It hasn't died all summer, but just started as the temp has dropped.

Any suggestions would be appreciated.

[RustyPile]

Need more information.
Does the choke work and is it properly adjusted?? What type of carburetor and choke?? Heated air, electric, manual???

[Steeveedee]

If it dies when you push in the clutch and things are cold, but not when things are all warmed up, I'd be looking for a weak or corroded connection, particularly at the firewall main connector.

[gmc396]

The carb and choke are stock to the engine; a Quadrajet carb and hot air coil choke. The choke operates and is set up correctly.

The wire connection at the firewall is interesting. Where should I start; which wires? Seems odd that all summer I had no problems but it started when the mornings are now cool.

[geezer#99]

Stock manifolds or headers?

[PbFut]

Sounds to me like Iike the choke may be opening just a bit soon. Can you rotate and put just a bit more tension on the choke spring coil and still get full open when at operating temp?

[gmc396]

all stock including exhaust manifolds.

[Sheepdip]

Originally Posted by gmc396

I have a '68 GMC 2500 with a 396 engine and granny 4 speed. The last few days have gotten chilly; down to the high 40s. Truck starts right up with one pump, and the choke sets. After I drive a few miles and engine is warming up, when I push in the clutch, the engine dies. I'm able to restart by letting the clutch out. Pulling into a parking space at the end of my trip (about 3-4 miles) it died again. When I came out an hour later, it started fine. This time on the way home, it did not die.

I rebuilt the carb last winter and performed a tune up. It hasn't died all summer, but just started as the temp has dropped.

Any suggestions would be appreciated.

Sounds like you might be "Icing" up which some choke adjustment should solve

[Steeveedee]

Originally Posted by gmc396

The carb and choke are stock to the engine; a Quadrajet carb and hot air coil choke. The choke operates and is set up correctly.

The wire connection at the firewall is interesting. Where should I start; which wires? Seems odd that all summer I had no problems but it started when the mornings are now cool.

If it dies when you push in the clutch, but starts and runs with the clutch pedal up, the clutch mechanism seems like a likely culprit. Or did you mean that it dies when you push in the clutch pedal, then you release the clutch pedal while in gear and it restarts?

[gmc396]

If it dies when you push in the clutch, but starts and runs with the clutch pedal up, the clutch mechanism seems like a likely culprit. Or did you mean that it dies when you push in the clutch pedal, then you release the clutch pedal while in gear and it restarts?

Yes, it dies when I push in the clutch and restarts when I release the clutch pedal while in gear.

[Steeveedee]

Originally Posted by gmc396

If it dies when you push in the clutch, but starts and runs with the clutch pedal up, the clutch mechanism seems like a likely culprit. Or did you mean that it dies when you push in the clutch pedal, then you release the clutch pedal while in gear and it restarts?

Yes, it dies when I push in the clutch and restarts when I release the clutch pedal while in gear.

OK, then the wiring comment does not apply.

[gmc396]

Over the weekend I cleaned the choke mechanism and checked all carb adjustments. Went to lean side with vacuum break adjustment. I thought maybe it was stalling because it was too rich.

This morning was 55. Got to my destination (2-3 miles) and engine died as I pushed in clutch to pull into parking place. Without touching the gas pedal, I cranked it and it started right up. It ran at the low end of high idle (about 1000 rpm). What I need to look at next time it stalls is where the choke is at. I wonder what would happen if I just removed the choke plate...

[geezer#99]

Fuel line is badly kinked right near inlet.

Being all stock does it have single exhaust with a heat riser?

[gmc396]

The kink is not as bad as it looks so never bothered to replace line. Plenty of fuel. Truck has dual exhaust with heat riser on passenger side. It is operational.

[geezer#99]

Maybe your crossover is plugged.
The dark area around the choke stove can indicate a plugged crossover.

[Sheepdip]

I'll still bet dollars to donut's your idle ports are icing up. Like geezer says check your cross over on your intake. Set your idle screw up just enough to leave the idle circuit and drive it and see if that helps

[gmc396]

Can you explain the blocked crossover? Is this a passage in the intake manifold? If so, I assume I'd have to take the carb and intake off to take a look.

Also can you offer more detail on "idle ports are icing up"? So I should screw in the idle mixture screws while still maintaining a decent idle?

Thank you both for your suggestions.

[kwmech]

If your 396 is anything like mine--after 10-15 minutes of running it is up to temp with the thermostat open and water flowing. So I think that would make the so called icing problem void. Good grief it's only high 40s out? Nowhere close to icing up in September. I'd be looking at fuel delivery, especially with that pinched/creased fuel at the bowl of the q-jet there. Check the rubber lines back to the tank. Maybe you just have a dirty filter inside the inlet. The clutch in and out makes no sense what so ever either than engine load which would come back to fuel delivery. Double check the points also. They may have closed up since last winter

[jeffahart]

I really think you are having ignition trouble. Have you looked around the clutch pivot bolt and z bar? they are right on the side of the block. An easy place to drape a wire and have it get yanked when the clutch is pushed in. Also, when you install the air filter and cap, it can relocate wires a bit without you noticing as it's pretty much right on top of the distributor. Bottom line, trace and check all ignition connections and re-crimp and ground; it will only take you a few minutes and you can check it off the list.

It's always easy to blame the carb first thing. Else, swap the carb with a known working part.

Good luck,
j

[Sheepdip]

Assuming your carb is clean inside, no vacuum leaks and engine timing is correct....You will need a vacuum gauge to properly/accurately set the idle mixture screws which I suggest doing also.
Hook the vacuum gauge to manifold vacuum, with the engine idling turn the idle mixture screws in or out, one at a time until you reach your highest vacuum reading, you may have to bounce back and forth to achieve optimum vacuum. Reset your idle rpm screw to 700 engine rpm's...Then your done.

[geezer#99]

Originally Posted by gmc396

Can you explain the blocked crossover? Is this a passage in the intake manifold? If so, I assume I'd have to take the carb and intake off to take a look.

Also can you offer more detail on "idle ports are icing up"? So I should screw in the idle mixture screws while still maintaining a decent idle?

Thank you both for your suggestions.

The exhaust crossover is a passage that runs right under the intake from passenger to drivers side.
Easiest test for plugging is a temperature gun. Look for different temp on both ends. Plugged will show colder on drivers side. Check this with a cool motor.

[geezer#99]

Originally Posted by Sheepdip

Sounds like you might be "Icing" up which some choke adjustment should solve

It’s not icing up!
More than likely fuel puddling in the intake.
Fuel vapor reverts to a liquid when it cools down.

[1967GMCC20]

Does it cut out quickly...or sputter and die.
Quickly would indicate electrical. I would look at coil. 12v leads. My 70 402 315hp runs like a sewing machine... and have not spent time on choke...it's 40s-50s here...no icing ever mentioned in these cold weather areas.

[1967GMCC20]

I see you have non HEI... check if converted to electronic, or still points. Check the resister block, or resistance wire voltage to coil. Also I would verify vacuum advance location on Q Jet..should be constant, not spark ported.

I run the similar high rise intake, but 1966 Holley version.

[1967GMCC20]

For a test...run a lead to + side of coil, bypass key with toggle switch. No improvement, not power to coil.

I've noticed fuel beginning to boil from heat caused by exhaust manifoldwhen sitting still idling. See it with clear fuel filter. Insulate fuel line with high temp sleeve.