Unburned gas can be a result of too lean of a mixture that failed to light when the ignition fired. Or a rich condition resulting in fuel that failed to burn when the o2 ran out durning the ignition event. Sence at that point all the o2 ran out, the only place the bang can get more o2 is from a leak in the exhaust system. If you are too lean and get a miss fire you now have all you need in the exhaust to get a late bang. Air with o2, some unburned fuel and heat
So now the solution. run close to stoch in overrun so that you run out of air at the same time you run out of fuel. The other thing that can help if you are running the later Extra codes is activate overrun about 2 sec after you roll out out of the throttle and turn the fuel back on in a stoch setting right before you get back to idle.
Hope this helps explain whats going on
AW