I have a 1980 Pontiac Firebird. It has a factory 301 with a Quadra jet 4 barrel on it. The 301 is a non turbo original to the car. I had the engine rebuilt about 2 years ago. I also removed the catalytic converter and put a test pipe on it. It was running fine but about 4 months or so ago it would snort when driving. Almost like a back fire in the exhaust or the carb. It is hard to tell but you can feel the snort thump in the floor boards when driving. 

I took the car into the Mechanic who did the rebuild. He thought it was an Ignition Module and put a new on on. That was not it. As he was test driving it, he said that it was when the car was up to temp that it started to do the snorting/backfire. He said it now is dying when the engine heats up and then will not start even with starter fluid until the engine is cooled down a bit. I have no idea and the car has been in the shop for 2 weeks now.

The car has had new exhaust very similar to the original put on it about 3 years ago, new radiator, fuel pump, water pump. There is almost nothing on this car that has not been replaced mechanically except for the Transmission which is all original to the car and shifts fine. The Mechanic said that he checked for spark and fuel and that both are present. The carb is the original Rochester Quadra jet that came in it and has been rebuilt in the last two years.

I am hoping someone can help as if its not fixable I will have to go with a 350 crate engine and that would be expensive to do. Thanks in advance for any help.