[johnnyrod]

Hmm my money on the backfiring - which I'm guessing is more of a pop than a gunshot - is a stuck choke and it's all running rich. Nothing to do with oil in the airbox - mine gets this now and again if I cane it, it builds up slowly really. You need to check the cable for kinks and make sure it has enough slack in it. Also check where it splits as this little box is easy to break. You could do with getting the plungers out and checking for rust etc. build-up on the seats. If you follow the cables to the two carbs, you need to undo the single screw of the retaining plate, and carefully pull out the plunger - if in doubt, pull the choke lever right back first and nothing will fall off. Check the recess for crud, repeat for the other carb. The screws and be stuck fast so use a good-fitting screwdriver and try not to chew them.