Another carb help...

[tim_mc]

The mention that it will idle, but drop RPM when put in gear AND aftermarket GM HEI caught my attention. I had a very similar issue which was an aftermarket GM HEI that wound up causing my problem: Truck had an occasional backfire or load up when driving. It progressively got worse to where it would start OK, but would die when put into gear. It would idle and rev OK, but ran badly under a load, or when stopping at a stoplight. I had to put it in neutral a few times to keep it from dying at intersections. Later it would lean out and backfire after it warmed up and eventually would not restart when warm.

Replacing the GM HEI coil and module with Davis Unified Ignition parts did the trick. No issues since June 2021. Coil and module are relatively cheap & easy to replace if you at least want to eliminate them as a possible culprit.

Also, I compared a stock HEI module out with a Davis module - what a difference in performance, especially acceleration.