You need a spark arrestor. The above statement is true if one of the exhaust manifolds or mufflers is cold or cooler than the other.
Unburnt gas igniting in muffs. Bad unclean carbs is also a possibility. My old quad used to do that to I think that when you let off the gas the rpm's are still kind of high and more air is getting to the engine and less gas and it kind of backfires sort of like but its not a bad thing I might have it backwards it could be running rich to.
bad carb setting/jetting or a timing issue . Read the plugs and see if the mixture is good , then check timing per manufacture instructions... if you have a upgraded pipe they always do that. it also could be running rich
a dirt bike
1989 Honda xr100
it is vary fast bike almost 60-50
55 mph or 60 own hill ...
get a bigger muffler
An older Honda XL100, XR100.
A Mongoose Mountaiin Bike brand new at The Bicycle Shop cost $350 in 2010
Put a different muffler on.
For trails XR100-XR650.For motocross YZ85-Yz125no that is not. the best is a razor electric dirtbike