Bulletin 9D-8 August, 1951 Model "BC" Page 8

To prevent stalling during the warm-up period, it is necessary to run the engine at an idle speed slightly higher than that for a warm engine. This is accomplished by the fast idle cam which is linked to the choke valve shaft and holds the throttle valve open sufficiently during the warm-up period to give the increased idle RPM, until such time as the choke valve moves to the full open position.

While the automatic choke is in operation, the driver may wish to advance the throttle to the full wide open position. Since this would decrease vacuum pull on the choke piston, thereby closing the choke valve, it is necessary to provide increased carburetor air flow by opening the choke valve mechanically.

To accomplish this, a tang on the throttle lever is made to contact the fast idle cam linkage at wide open throttle position so as to partially open the choke valve

This will also relieve excess choking on starting by allowing more air to enter the carburetor when the engine is cranked with the accelerator held fully depressed.