1971-1973 Arctic Cat Snowmobiles Factory Service Manual, Page 93Get this manual
CARBURETION (TILLOTSON) Idle Operation
When the engine is idling, the throttle valve is partially openDuring the idle process, engine suction is transmitted through the primary idle discharge port to the fuel chamber side of the main diaphragm via the idle fuel supply channelAgain, the main diaphragm is forced upward by atmospheric pressure, depressing the inlet control lever and permitting pressurized fuel to force the inlet needle off its seat, filling the fuel chamber side of the main diaphragmThe fuel is then drawn up through the idle fuel discharge portNOTE: The entire carburetor is at atmospheric pressure during idle operation