Recently rebuilt the top end of a small block chevy 350. I am having a problem with the engine backfiring through the carb when the secondaries open on the carb. I was using an edelbrock 600. I tried everything from tuning the carb with different metering rods, jets, springs but nothing got rid of the backfire. I played with the timing thinking i needed to advance more but that didn't solve the problem either. Being completely lost and having nothing to blame i bought a new holley 600 carb thinking it was the edelbrock giving me this issue. I still have the same problem. The engine fires up great, idles fantastic, revs with alot of response and can cruise down the road fine. Once i go Wide open throttle it backfires with a flame right through the air filter. The total timing is right on 35 degrees. I didn't change anything with the holley yet. What else on the engine can i check that would cause this to backfire? I can go out and change the wires, one got the coating melted a little and is now taped, would that cause this? The truck has hydraulic lifters and roller rockers.When i installed the roller rockers and started to tighten the nuts , i went slow till i felt a slight drag on the pushrod then went a 1/4 turn past that. Is that not enough preload ? This is my first 350 engine rebuild. Thank you