It has doing this sense I installed the engine, started with a points distributor and change to MSD, when I first installed the engine it would backfire throught the exhaust so bad it blowed the mufflers out so I install a set of flowmasters and adjusted the valves,change the distributor, adjusted the carburtor and set the timing, it still will backfire on start ups and when you press the pedal to the floor it will stall out, could it be the power valve in the holley or the ignition switch, I am lost, any advice will be thankful, later