Gas engine with F.I. or carburetor will still be allowing fuel into the combustion chambers, it will be only enough fuel for idle but as HRL has noted, that is a very lean mixture and produces backfiring on pretty much every exhaust stroke. If you were able to kill the ignition OR completely cut the fuel on coast-down, you won't have the noise. Next time you are coasting down, keep your foot off the throttle, cut the ignition and hear the difference, just don't turn the ignition back on while the rig is still coasting down, you'll be buying new mufflers.
