Just a thought. I've read a few posts here and many have a " backfire" associated. I'm leaning, ( excuse the pun), on a lean condition. Todays fuel holds ethynol, which when combined with time and direct venting to the atmospere, ( bike, boats, lawnmowers,etc.), can seperate into two different, non-combustable forms of alchohol. This is called phase seperation. I would think a good carb clean is a great place to start. There's nothing wrong with a good cleaning anyways.