1970-1979 Ski-Doo Snowmobiles Service Manual, Page 46Get this manual
TROUBLESHOOTING
Figure illustrates the next phase of the cycleAs the crankshaft continues to rotate, the piston moves upward, closing the exhaust and scavenging portsAs the piston continues upward, the airfuel mixture in the cylinder is compressedNotice also that low pressure area is created in the crankcase at the same timeFurther upward movement of the piston uncovers intake port (D)A fresh fuelair charge is then drawn into the crankcase through the intake port because of the low pressure created by the upward piston movement