Hate to answer a question with a question but, do you really mean backfire, or are you hearing a burble?

Also, what brand/size prop? Hope you're not underpropped........

Assuming you're not, then....
If it's backfiring it can only be related to ignition, and most often ignition timing.
I suggest you check it, and perhaps retard it slightly.

If this doesn't work, then it's back to the distributor, unless you just simply want to replace the ignition module.

Good luck....