The answer is very simple. It is because they can, backfire, burp, pop, etc, etc, etc, and when they get tired of all that, they throw a whole lot of fuel into the exhaust and out comes a wooping sheet of flame. FAN B****Y TASTIC.
Backfiring happens when the throttle is closed, it is the unspent gases igniting out of the chamber. I think we tried to explain this a few times!
