Re: backfire
When was a thorough tune-up done last -- if ever. By thorough I mean carb rebuild, new plugs, points (if used) decarb, timing and link & sync. Could be a partially sheared flywheel key. Sneezing (spit back through carb) and backfiring are two different things so if the engine is simply sneezing a little at idle, carb adjustment may take care of it. A bad reed(s) can cause sneezing.