Abstract: A system of controlling the exhaust and scavenge process for two-stroke piston engines with crankcase scavenge and fed by air/fuel mixture, which has a rotary valve adjacent to the cylinder exhaust port/s that rotates in synchronism with the crankshaft enabling the control of the exhaust gases initial speed and preventing the air/fuel mixture from leaking to the exhaust system during the compression stroke; this way, the engine power is improved, and the CO and HC emissions and oil and fuel consumption are reduced.