Just to labour the point, overrun the popping and burbling in a harley or most twins will not harm your exhaust, a bad fuel mixture caused by swithching of the spark leaving too much unburnt fuel in the collector pipes can, it just like making a rolling accetelane bomb for those with a misspent child hood, all petrol combustion engines naturally burble, you dont hear it in a car due the amount of silencers fitted and physical length of the exhaust,
If you try to alter the natural (allowed for in the design) scheme of things then you CAN not always will cause your self problems, loud bang and pop=pressure, normal burble is dealt with quite happily by the diameter of the pipes.
It doesn't happen on throttle because the fuel mixture is burnt as part of a well timed cycle, on overun however this cycle is absent and the mixture burns at will when there is either enough present in the exhaust or ignition at a random point. 4 stroke (otto) cycle:- suck, squeeze,bang and blow, all in the engine its fine, unfortunately there is no easy way to control the bang and blow in the exhaust.
Ps cerruti Harleys are normaly a little louder because
a/ the exhausts are of a bigger diameter.
b/ they normally have a straight through configeration
c/ they are large Twins.
