Ya' learn something evey day. I was doing some reading, and came upon this.

"This is a very common misnomer in engine circles! What you are experiencing
is an after fire, not a back fire. A true back fire would have gases blowing
back through the induction system - NOT a good thing! I don't know why
the phenomena you are experiencing has become commonly known as a 'back'
fire, but there you go. <g> I would guess that your weak inlet valve spring
is the most likely cause, as the engine must be drawing some fuel in when
coasting and this is no doubt collecting in the exhaust. To rule out exhaust
restriction try running it with the muffler removed and just a short length
of pipe in its place. The second most likely is a sticky exhaust valve when
hot. In more sophisticated engines there can be other causes for after
firing, such as incandescent carbon, late valve timing etc., but I think
your problem is much simpler."

Regards
Brad