i rebuilt a chevy 350 for a 78 chevy blazer about a year ago i have never been able to get it to run 100%, this morning i discovered that modulator in the distributer was bad on both low and high rpm, i went and replaced the distributer with an accel. now i can get the engine to stop backfiring, where before it would just knock on acceleration and lacked power. if anyone know where to start i'm open to ideas!!! carb is a holly 600 with vac. secondaries i have had problems trying to adjust this carb could it be the problem?  

Hey Buddy, I jsut saw your message! I don't know if you solved your problem but if you have not here is something that might help? I just started my 350 again after a year, every part is brand new and I also run a holly 600 and accel distributor. After starting it again I noticed it had a mis and was running rough, now knowing that everything is new and the only time I ran it it ran like a champ I became confused? Why would it be running so bad, you give it some throttle and it back fires thru the exahust what the heck is going on? well what I found was the problem with mine was I had a vacume leak in the manifold because after sitting so long the gasket lost there seal, thiscould also accure atthebase of the carb etc... vacume from the intake will make it backfire. Also check the plugs they can become fauled just from sitting after you run it for a short period. There are only a few things that can go wrong here is a list to check.

Plugs
Plug wires
vacume leak at manifold or carb base, fittings etc...
Faulty accelerator pump discharge
timming

Let me know if this helped?