Hello, please help me!!! I'm trying to modify my 350 but i dont know where to start! What should i do in order to keep from throwing things out of wack? Right now i'm running a 750 holley and have had nothing but problems with it. I've blown 2 power valves and have replaced gaskets and it still backfired. should i replace the carb?  

Either the engine is not timed properly or the carb is running lean.

A 750 Holley is a bit much for a 350 unless it's really built and can turn revs above 6500. But 750s have been done successfully on street 350s but it takes some work.

To get into fixing it I'd need to know a lot more about the engine and vehicle. Cam timing, igniiton timing, compression ratio, air cleaner, which cylinder heads, type of spark plugs and wires, type of intake manifold, exhaust heat or not applied to the manifold and or incoming air, type of exhaust system, transmission, final gear ratio, vehicle weight, tire size all are players in how the carb responds.

This doesn't take into account the effects of a vacuum leak, blown head gasket, dying fuel pump, plugged filters,stuff like that.

Bogie  

id get a smaller carb, bout 550 or 600cfm,an alunimun intake, hotter plugsand go from there  

I've run a Holley 750 for years on my small block without a problem. I did have it tuned by a Holley Carb guru shortly after I installed it. He also trued the mounting flange on the carb. He said a lot of problems he's seen with the holleys are the mounting flanges do not seal correctly - not trued flat from factory - shocking, I know (insert sarcasm) that causes air intake leaks when you stand on the skinny pedal. I haven't had a problem with it since.
If I were to do it all over I'd probably get a 670 Holley or a Demon carb in that cfm ballpark.
750 cfm carb are intended for 1/4 mile cars by design.  

thanks guys! I know that these carbs are impossible to tweek yourself unless you know what you doing so maybe i'll have it done and see what happens. i'm also having problems with the choke too, could that be a possiblity of poor performance?