All sorts of stuff!
question 1) Any carb or exhasut mods?
question 2) Did you check your plug?
edit: and by this I mean, see if it's fouled, could be caused by sudden float mechanism and seat failur, petc0ck failuree... then check to see that it's sparking.
question 3) (not a question) Don't use premium gas, use the lowest grade that doesn't cause "pinging." edit 2: The stuff I use is not much better thtn pee after a good night of drinking!
question 4) Did you check for exhaust leaks?
I'm sure others will chime in...
oh, here's a tip, backfiring is a common problem, so some folks don't reply, or reply snidley and tersely (Usually not a problem...) so put "NEW" or "SUDDEN" in the title.