Well actually, it is NOT really backfiring. We get a lot of posts here like "my bike is backfiring, what can I do about it?" So maybe this sticky will help clear some thngs up. Backfiring, is actually fairly rare these days because of electronic ignitions and the high reliablilty of engines, and the lack of things we can actually mess up on an engine. Real backfiring means the engine is out of proper timing and the fuel/air mix is getting a spark and ignition at the wrong time, when the intake valve is open. Real backfiring is actually the bike "spitting up" in a sense, backwards through the carb or intake.
So, the popping you are hearing is not back-firing. It is a popping resonating sound generated by a lean condition on decel. When you suddenly close the throttle, the incoming air is choked off and the closed throttle also causes the bike to return to idle on the very lean idle circuit of the carb(s). However, momentum of the bike is actually "driving" the engine in a sense, and that engine is still spinning at rpm that needs far more gas than the closed throttle/carb can provide. The engine is still spinning but the incoming fuel rate is very low, until just enough builds up in the combustion chamber to "fire" on the exhaust stroke of the engine.
