For the most part, there are two types of 2 stroke engines... reed valve, and rotory valve. Our little nitro engines are rotory valve.

These engines have a two stage intake tract. First stage, piston is on its way up transfer ports are closed, vacuum is created in the crankcase. Then the rotory valve opens (crank window)... and the fuel mix is drawn into the crankcase.

Second stage. Rotory valve closes (crank window), piston is on its way down.. pressure builds in the crankcase, the transfer ports open.. and the mix is pushed up into the cylinder.

This is all happening at extreme speed ... in these little engines.

If you get crazy grinding on that crank window... that pressure thats building in the crankcase will take the path of least resistance... and blow back out the carb... instead of filling the cylinder.