350 dies when warm and won't start

[andrew-75c10]

Background: I've been working on my 75 C10 that I got from my grandfather. So far I have replaced the fuel tank, fuel pump, and rebuilt the carb. A couple months back I bought a tach and got the cold idle and warm idle RPMs dialed in. I also bought a vacuum gauge so I can get the idle mixture screws dialed in. However, a few minutes after my truck warms up it dies I cannot restart it.

At first I thought this might be a cooling problem (none of my gauges work but one problem at a time). I was talking to my dad and he said I should really changed the coolant either way. So today I did my first coolant flush. I was expecting my coolant to look like dirt after watching how-to videos on youtube. It still had some green color, but it wasn't as bad as I imagined.

After replacing the coolant, I hooked up my vacuum gauge to the brake booster line. When I start my truck cold, I get 20 PSI of vacuum. As it warms up, the vacuum drops to 15 PSI. If I turn off my engine at this point, it won't start again. If cooling isn't the problem, I'm stumped.

After reading the forums, I was going to try:
1. Get a clear fuel filter so you can see if the carb is getting gas
2. Get a spark tester

Hopefully I can figure this one out so I can get the rally wheels that I bought installed

 

[Ricko1966]

Forget both of those items look down the throat of the carb open the throttle by hand fuel should squirt 2 streams like a squirt gun down the throat, if it does it's not a fuel problem. If it doesn't then wecan start trouble shooting fuel. The inline spark tester will light with a lightning bug fart, but not enough spark yo start an engine. Take a spark plug open the gap to about .080 clamp it in a jumper cable clamp the jumper cable to ground put a plug wire on the plug and crank. That will tell you if you have spark. Post results.

 

[80BrownK10]

Is your coil good? Ignition module. Both if get hot could be failing. But I wouldn't think it would happen every time, every now n then maybe.

 

[80BrownK10]

Yes like said look down the crab when you open the throttle. Quick test if it's fuel issue just take some carb cleaner and spray into the carb for 2 seconds then try to start it. If it fires you ha e a fuel issue if not, probably something else, like ignition related.

 

[Rusty Nail]

Take off the gas cap.

 

[75gmck25]

First question- what carburetor are you using? If it’s the original Quadrajet, what is the number on the side, and what rebuild kit did you use?

Buying Quadrajet rebuild kits can be confusing, since GM switched to a newer style carburetor on the trucks later than they did on cars. If you buy a rebuild kit for a 1975 Quadrajet it will probably be for a 1975 car, which is the wrong kit for a 1975 truck. You must go by the number on the Quadrajet, not the year of the vehicle. For example, the original carburetor on my ‘75 K25 is number 7045583 YZ 3454, which was used on HD emissions trucks.

Bruce

 

[andrew-75c10]

I have some starter fluid, and after it warmed up and had trouble starting, I spraying some in there and the engine would turn over. I started looking for fuel problems, and low and behold I found my accelerator rod was bent. I'm really not sure how this happened, but maybe I need to find a new one. I straightened it out the best I could for now.

My truck seems to idle fine now, although I noticed the vacuum fluctuates between 9-13 mmHg. I'm guess it means I have a vacuum leak, so on to the next problem.

Here are some pictures of before (I already took it after the carb) and after (installed back on the carb).

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[andrew-75c10]

I spoke too soon - made it a few times around the block then wrote up this post. I tried to go around the block again and it did not make it.

 

[Rusty Nail]

Let it learn ya.

 

[andrew-75c10]

As far as I can tell this is a fuel problem. Which makes sense since the vacuum gauge is so low that I'm probably not getting any fuel. Guess the next step in the switch hunt is to look for vacuum leaks.
https://www.hotrod.com/articles/hppp-0704-how-to-identify-engine-vacuum-leaks/

 

[Raider L]

@andrew-75c10,

Q-jets are nortorious for needing perfect vacume. I had these problems and it could very well be a vacume leak internally. Make sure if you have any rubber caps or rubber hoses attached to it check them for cracks or looseness of a hose. Replace rubber caps and any hoses that look suspitious.

Otherwise i am leaning towards the coil. Except the coil isn't going to be getting "warm" like the carb is. So it's probaby the Q-jet. Want to solve the carb problem? Buy a Holley!!

 

[idahovette]

@andrew-75c10,

Q-jets are nortorious for needing perfect vacume. I had these problems and it could very well be a vacume leak internally. Make sure if you have any rubber caps or rubber hoses attached to it check them for cracks or looseness of a hose. Replace rubber caps and any hoses that look suspitious.

Otherwise i am leaning towards the coil. Except the coil isn't going to be getting "warm" like the carb is. So it's probaby the Q-jet. Want to solve the carb problem? Buy a Holley!!

This is not necessarily the answer, just get your Quadrajet set up correctly and you may be fine!