1981 Z-28 put in a crate 350 vortex. Have installed 4bbl quadrajet. I unplugged the computer a while back car ran fine. I should mention it has a vacuum advance HEI dist. Car sat up for a while after wife passed. I ran it every month or so. Recently went to put car on road and it was running something awful. Okay I figured things got out of wack with the humidity here in S.E. Louisiana. Put in new Dist., plugs, re-did carb., fresh battery and all the rest. Swapped out coils with my 85 suburban just in case and it worked fine.
As soon as I start the engine it is running ragged. Within about a minute after warming up it starts to pop then the backfiring starts to get progressively worse. I shut it down immediately and continue looking for the problem. I have been working on this problem for about a week now with no luck. Any Ideas as to what I am forgeting to do? Appreciate any insights. Thanks

 

Is it popping back through the exhaust or the carb? Assuming that the ignition and carb are good, then there may be a mechanical problem, such as the timing chain jumping teeth, or a valve hanging open. A compression check would help to pin things down a bit.

 

Since you've unplugged the computer, did you replace your computer controlled distributor and carburetor with stand-alone units?