Help with troubleshooting C20 350 that dies after short drive

[vadriller]

I am in the process of getting my truck running after it sat for 20 years.

I am at a loss and need some help. The truck was running fine two weeks ago...now this.

The truck will fire up quickly and idle all day long (20 minutes fine) when I go to drive it, it runs for about four or five minutes great. Then skips a little and the engine dies. I can crank and pump the pedal for a while but it will not fire. If I let it sit turned off for about three minutes, it will crank up perfectly and run for another couple of minutes and die again.

Here are the specifics:
74 C20 with the 350 and Rochester 4 barrel
New fuel tank, fuel lines, and fuel pump
rebuilt carb (<1 month ago)
new ignition coil, wires, spark plugs, points and condenser

Here is what I have tried so far:
New (2nd new one) ignition coil
New (2nd new set) points and condenser (dwell set at 29 degrees)
pulled out the fuel filter and it looked clean
when it stalled, I opened the fuel cap and cranked for a while, but nothing
when it came back to life after sitting, I left the fuel cap cracked open, but it still died

Any suggestions about what to try or test next?

 

[DoubleDingo]

Maybe bad gas? Gummed up carburetor?

 

[vadriller]

I don't believe that to be the case. The fuel system was brand new before the rebuilt carb was installed. I just checked the fuel filter and it looked clean.

I am having problems wrapping my head around the fact that the issue briefly "cures itself" after sitting for a couple of minutes.

 

[1987 GMC Jimmy]

Have you verified good spark whenever this happens? Have you checked for vacuum leaks? Base gasket, emissions equipment, those types of things...

 

[Frankenchevy]

Verify float adjustment

 

[Frankenchevy]

The floats are probably shutting off the inlet with barely any fuel in the bowl. A small amount of fuel on reserve can allow for idle and a short trip but will quickly run out when you get it up to speed and keep it there.

 

[Frankenchevy]

I’m not sure of the float clearance on that carburetor but mine is about 3/4 of an inch. And they come back from being rebuilt out of spec all the time.

This would explain why it ‘cures’ itself, as when you come to a stop the float bowl can replenish itself.

I would put money on that or your fuel turning into vapor in the lines before it makes it to the carburetor. Make sure that the metal lines in your engine compartment are far from heat sources if possible and you can always insulate them if you don’t know how to move them properly.

If neither of those fix it, I’d start looking at the fuel pump and the carburetor whether or not they are new or rebuilt.

 

[vadriller]

Thanks for the advice everyone.

Here is what I have tried so far. I took the air horn off and the float was supposed to be set at 11/32" and if it was off, it wasn't off by much, I got it set to where it was perfect before reinstalling. Same result....

I moved my attention to the fuel pump, it is giving 9 psi at the carb and fills up a little more that a pint in about 10 seconds.

I went out and bought a carb rebuild kit and took it apart. Dipped and sprayed everything. New fuel filter. All the passages are open and clear. Put it back together and carefully adjusted the float. Same result.

It fires up quickly, runs good for a minute or so, then slowly dies. I had my wife come out in the cold and turn the engine over and it had spark right after it dies.

What else can I look for?

 

[Frankenchevy]

have you watched to see what the choke flap does as it warms up? have you pulled the plugs after it dies and smell raw gas in the cylinder?

 

[vadriller]

The choke flap is opening like after it is warm. It does open up relatively quickly. By that I mean, the upper radiator hose has not opened yet and the choke is all the way open.

I have checked the plug on the number one cylinder after it died and it did not seem like it did not have the smell of raw gas and definately was not wet. When it fires up again, it does not act like it was flooded either (no black smoke or anything like that).

I keep wanting to go back to electrical (I think I am just sick of inhaling gas vapors today). Is there anyway I can jump power to the coil....or do I need some type of resistor for that?

 

[roundhouse]

After it dies. Take off the air filter and look down on the carb with a flashlight

Pump the throttle linkage
Does it spray gas ?


Install the pressure gauge so and tape it to outside of the windshield so you can watch it while you drive

Rubber lines from tank could be collapsing

 

[1987 GMC Jimmy]

Nine pounds is a bit much for a mechanical fuel pump. That may not be the problem itself, but it’s worth noting that five is more ideal. I think someone on here did a regulator because their fuel pumps kept overdoing it. Was that you @spanky55amg? He should be able to tell you his symptoms, and see if there’s any similarity. This is strange, though.

 

[vadriller]

I should note that the pressure was measured with a harbor freight fuel pump and vacuum gauge. All I did was press the tip of the gauge tightly into the end of the metal line where it goes into the filter on the carb. Then my wife cranked it over until the gauge didn't climb anymore. I don't know how much stock I put in a $10 tool, but it did squirt all over the place when I removed it from the line.

I am starting to feel like I am chasing my tail on this one.

 

[1987 GMC Jimmy]

What about the vapor lock concerns? Are you running the factory steel line from pump to carb? What temperature is your engine running? I’m sorry you feel like you’re chasing your tail. I think everyone feels that way at some point, but it’ll come to you, and you’ll get it. Was it doing this before you replaced the points, condenser, and coil?

 

[Rusty Nail]

9psi is too high.

I'm not convinced your problem is fuel related. I should go back and read again..