"This post is copied from yaho
"This post is copied from yahoo.com answers:
If you go to How things work.com they have some good flashes that show how a 2 stk works. A reed valve allows air to be pushed in through the carb or throttle body in to the crank case when the piston is moving from BDC (bottom dead center) to TDC (top dead center) the end of this movement is the secondary compression stroke. This movement also allows the crankcase area to increase and the pressure decreases thus allowing air to be pushed into the crankcase past the reeds. As the prior air/fuel charge is ignited, the expanding nitrogen pushes the piston from TDC to BDC thus creating primary compression of the air fuel mixture in the crankcase. This will then push the air fuel mixture into the cylinder via the intake ports in the cylinder wall. The reed valves must seal the crankcase pressure or the mixture will be pushed back through the intake (carbs) and then "spitting" fuel outward. Reeds are a one way check valve,in a nut shell.
The running issue will vary as the degree of failure. Split or chipped reeds will cause a running issue at higher speeds, not too much a lower speeds. Broken peddle will cause low speed missing and no running at higher speeds. Weak reeds will make a motor run great at low speed but limit the rpms the motor will turn. A damaged reed will not cause a backfire, it can do what is known as a lean pop through the intake.
If you have an Optimax, different running issue, let me know.
To clear something else up, 2 stk motors control the crankcase pressure through, piston port, Reed valves or rotary valves. Reeds can either be plastic or metal"
