[randy500]

Could an intake valve be slightly open which would introduce flame to the intake and cause a backfire?
Check this with a vacuum gauge at idle the gauge should be steady, if it fluctuates quickly then an intake valve is open. Make sure the gauge you use is not highly dampened, some gauges have a tiny hole which restricts vacuum and smooths the gauge reading, I have a sun vac gauge that is useless as it has a hole which is about .010 in diameter or smaller, its tiny