Fuel starvation can be a problem. Essentially backfires can be caused by improper fuel/air ratio, bad timing or a spark plug that for one reason or another isn't firing as it should...not in that order nor in any particular order. Bottom line is you're getting unburned fuel in your exhaust manifold (or even in the pipe leading away) and it is being ignited at the wrong time and in the wrong method. I've even seen a vacuum leak cause backfiring.
