455 aluminum headed Firebird. Fresh from the rebuild and have put around 50 miles on the motor. After I got my carb adjusted I decided to hook up the distributor advance can to full manifold advance. The car started better then it ever has, no cranking. Got it warmed up, went to get out to adjust the curb idle speed and the carb backfired and died, no sooner than I opened the door. Unhooked and plugged the vacuum line and the car would not idle. Since then I have replaced the power valve. I did observe that BOTH sight glasses had the fuel half way up after I adjusted them to the bottom, with the strong odor of gas. Also, a day later I checked the carb and the fuel was at the bottom of sight glass.
What would cause this random backfire? Is it possible both needle and seats had crud?, bad floats? Also, my proform 750 has blowout protection, would this still allow a blown out power valve? I am stumped as to the backfire.