1969 Ski-Doo Snowmobiles Service Manual, Page 6Get this manual
charge is then compressed when the piston goes upward with the second strokeThe fuel charge is ignited by the spark plug just before the piston arEives at the top of this strokeThe fuel charge does not explode, it burns and spreads across the combustion chamber so to give an increasing pressure which drives the piston downward as it comes past top dead centerThis produces the third stroke of the piston which is the power stroke, caused by the expansion of the heated gas which shoves the piston downWhen the piston reaches the bottom of this third stroke, the exhaust valve opens and the burned gas is pushed out of the combustion chamber by the piston corning up for the fourth strokeAt the top of this stroke, the exhaust valve closes, the inlet valve opens and the piston is ready to begin new cycle Intake