So, the popping you are hearing is not back-firing. It is a popping resonating sound generated by a lean condition on deceleration. 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. 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 position 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.
Don't "change the plugs" like some people suggest, it’s a waste of time and money. Messing with the ignition timing really won't help either, because it is still a lean fuel/air mix on deceleration that is the cause. ( I said cause and not problem, because the popping is not at all damaging to the bike).
Often people think their bike is not tuned properly after the addition of “aftermarket” mufflers because of the additional popping they hear. Stock mufflers provide a lot of volume for the rumbles and popping in the exhaust to be toned down by resonating in the larger muffler canister. Motorcycle builders also have to meet sound regulations along with emission standards. Some of that sound proofing is in the mufflers. But, aftermarket mufflers that offer easier breathing, and are usually lighter and smaller, don't offer that same volume (Less volume = less sound attenuation = more noise and more popping). Modifying the exhaust (especially if it gets louder) will only make the popping louder. Because most aftermarket pipes produce "lower back pressure", to add to the popping; aftermarket pipes also tend to lower back pressure this will cause the fuel/air mix to lean out. So don’t worry about that popping you sometimes hear and enjoy this year’s riding season.
Wild Fire Harley-Davidson