I have an aftermarket chevy 350 engine in my 68 camaro. It has iron Vortec heads, hedders, a 750cfm double pumper 4160 holley carb, and a hydraulic cam and has been running great until today. It started backfiring out of the carb when I rapidly throttle up. It occurs around 2000-2500 rpm. I have already checked the firing order, ensured #5 and #7 spark plug wires do not touch each other, checked the valve lash, and pushrods, idled the engine with the valve covers off, and I don't see any problems. Also, if i smoothly bring the engine up to speed, i can max out the rpm's without a backfire. It only happens when I rapidly advance the throttle. Could a valve, lifter, or pushrod be hanging up? Or could the timing be off? Any help would be greatly appreciated.