What might cause the engine to pop, kinda sounds like backfire from most cylinders, when i let off the gas , like coming to a stop, but idles fine and no hesitation on accelaration or misses,otherwise runs fine except when i let off gas then pop pop pop,before you ask heres what i did, got heads redone,set valves the way the book said, by cranking the bolt till you can barely spin the pushrod, then 3/4 turn,set the distributor to #1, set timing to 8 degrees btdc, brand new edlebrock carb , no adjustment made, checked for vac leaks none to be found, distributer vac hooked to non manifold port on carb