I would suggest a tune. It's my understanding that a backfire is caused from unspent gas in the exhaust system that ignites from the hot pipes.. and of course any fire or explosion requires a certain amount of oxygen, like what BigWillie said. A good tune might be the solution here.

 

i unplugged my airpump and it no longer backfires at all! is it bad to have an unplugged airpump?