It sounds like you have a vacuum leak or a problem with the choke circuit.

With the engine running listen for air being sucked into areas around vacuum hoses, rubber intake hoses, etc. Move them around also as it might make the noise more apparent. I also use a string and drag it around the area where I hear the vacuum leak. I just did a timing belt on a car and cleaned the intake. it had a vacuum leak and I replaced a few vacuum lines that were suspect because they were hard and brittle as rubber hoses will do over time.
I was dragging the string near the throttle body where it meets the intake manifold. The string got pulled towards the place where the 2 meet. I pulled of the throttle body and I had put the gasket on wrong and a vacuum circuit was not sealed. i cleaned it up and put silicone on the gasket, which was high quality enough to reuse, and it sealed up and took care of the issue.

After looking for vacuum leaks, check that each cylinder is running. You can do this with a IR temp gauge, the type that you point and shoot at something to get the temperature. See if the cylinders are the same temperature. Another way is to get some water and drop some on each part of the exhaust manifold near the exhaust port of each cylinder. It will boil off and if one cylinder appears to boil off more slowly, then there is another issue that we can get into if that is the case.

There are some good links on this site to diagnose the issue, but this is a good place to start.

Also, is your car carbureted or fuel injected? I am assuming carbureted. If so, check the vacuum diaphragm for the choke. As the motor warms up, it should change position as the motor reaches operating temp.

Let me know how it goes.

Answered by DrCranknWrench (expert)
3,292 answers provided

Good afternoon, I own a 1995 Corvette with the 5.7ltr fuel injected engine. My car is doing exactly the same thing. Starts and runs great when the engine is cold, but is very hard to start when it is warm. If let set 5 - 10 minutes, when it does start, it stumbles, knocks and pinmgs and most time dies. It will eventually start up and run perfect?

Answered by davidgooch
2 questions asked

That sounds like the ?Fast Idle Throttle.This is the fuel unjected equivelewnt of a choke. Sometimes Idle Air Control valves can go bad to. they add more throttle when the engine load changes.
You might have a dirty throttle body and intake which require removing it and cleaning the posrts for the FIT and IACV from the back.

Answered by DrCranknWrench (expert)
3,292 answers provided