There are two common causes of 2 strokes spitting back through the carburetor: blocked exhaust port or muffler, and lean mixture. The difference between the two is that if the mixture is lean, the fuel will be expelled violently and will probably be burning (known as a "spit", and quite noisy when it happens). However you should also inspect the sides of the piston (through the ports) for scoring. A piston scored on the intake side does not shut the intake port off properly, and it will puff back a bit during crankcase compression.
