When the engine is cold the tension of the Thermostatic Choke spring (10) holds the choke piston (2) at the top of its travel and the choke valve completely closed. This supplies the engine with a rich mixture for starting.
When the engine starts, the vacuum of the intake manifold acting upon the choke piston and the unbalanced choke valve partially opens the choke valve until it assumes the position where the tension of the thermostatic choke spring is balanced by the pull of the vacuum on the piston and valve.
As the engine warms up slots in the sides of the choke piston cylinder allow the vacuum of the intake manifold to draw warm air from the exhaust manifold stove through the tube (7), through the choke air cleaner screen (8) past the thermostatic spring (10) and into the intake manifold through the passage
This flow of warm air heats the thermostatic spring and causes it to decrease its tension. The pull of the vacuum on the piston working against the de, creasing tension of the spring, gradually opens the choke in such a way that it is fully open when the engine is warm enough to run on the regular idle mixture.
If, during the warm-up period, the engine is accelerated, the corresponding drop off in vacuum, which automatically comes with acceleration, allows the thermostatic spring to momentarily partially close the choke, providing the engine with a mixture rich enough for acceleration.
Since on wide open throttle at low enigne speeds the intake manifold vacuum drops to practically zero, it is possible, at the beginning of the warm up period while the thermostatic spring still retains some tension for the choke
to be closed by the spring, thereby causing an excessively rich mixture. To prevent this, an arrangement is made in the choke linkage so that on all wide open throttle operations the choke is held open. (Called the Lockout).
During the warm-up period, it is necessary to run the engine at approximately 12 to 15 m.p.h., noload speed to keep it from stalling. This is accomplished by having the high spot on the cam come under the idle speed adjusting screw, holding the throttle open sufficiently to provide the necessary engine speed. This is called the Fast Idle.
When the engine warms up sufficiently to run at
regular idle speed without stalling, the operation of the choke moves the cam out from under the idle speed adjusting screw.
If, for any reason, during the starting period, the engine is flooded, it is necessary to be able to hold the choke open sufficiently to allow the engine to clean the excessive gasoline out of the intake manifold. This is accomplished by an arrangement of the throttle lever and choke linkage, whereby depressing the accelerating pedal to the floor board forces the choke open sufficiently to allow the engine to clean out the intake manifold. This device is called the Unloader.