Ok I know things like this have been posted a lot, but i couldnt find anyone in the exact same situation like Im in.
(1985 12A)
I can be in nuetral, 1st, 2nd whatever...at any speed...at any RPM's under like 4500...If I jam on the gas my RPM drops down real low and if i hold it will sometimes stall out. I replaced the fuel filter last night...didnt help ( although it seemed to make my RPM's rise smoother ) but didnt solve my main problem. I am thinking its the fuel pump since I did upgrade to Holley 450cfm carb, and those take more gas than the stockers.
If anyone has had this problem before or you think you know what the problem is PLEASE let me know...its really annoying. Thanks in advance ~Tim

 

I had the exact same problem. In my case it was the fuel filter. You definatly have a fuel flow problem. Perhapse your carbureator is not tuned correctly, or there is a damaged fuel line, and fuel is not getting to the engine. If it is not those, I would say fuel pump. good luck.

 

Check fuel delivery first. Volume should be strong, and IIRC Holleys like about 6psi pressure. Also do a quick check of the accelerator pump, make sure its squirting.