Popping on the over run is down to a lean mixture AFAIK, the unburnt exhust gases don't burn in the exhaust itself and build up resulting in a loud POP! when they finally ignite I've leaned off the low end fuelling on my turbo bike by a fair bit and it now sounds like WW111 going on behind me when I throttle off. Kind of pisses you off after a while and will do no good to your exhaust internals.

Mark