The automatic choke control is of the hot air type. Its operation is based upon the combination of intake manifold vacuum, an offset choke valve, and a thermostat coil spring, which is controlled by heat from the exhaust manifold. The vacuum operated piston is connected directly to the choke valve through a connecting link. The thermostat coil spring, which engages a pin on the connecting link, controls the operation of the choke valve under varying temperatures and throttle positions. The heated air which governs the tension of the thermostat spring is supplied through a heat tube which connects the thermostat cover with the exhaust manifold “stove”. The fast idle cam operates in conjunction with the automatic choke to provide the correct throttle opening and increased engine speed to prevent the engine from stalling during warm-up.
Two types of fast idle cams and cam levers are used on the series WW carburetors (see image below). Both types are mounted on the end of the choke shaft and are controlled by an extension on the throttle lever. In the design shown at the left, the external tooth fast idle cam is illustrated. The entire fast idle mechanism is enclosed by a cover plate attached to the side of the air horn. In the unit shown at the right, a large quadrant type of fast idle cam is used. In this design the throttle stop screw is located at the outer end of the throttle lever. A button at the end of the throttle stop screw rests against the fast idle cam whenever the throttle is closed.
As the engine cools, the thermostat spring also cools and gradually gains tension. The thermostat spring however, is unable to close the choke valve until the throttle valves are opened sufficiently to move the throttle stop screw away from the fast idle cam. The tension on the thermostat spring will then close the choke in accordance to the prevailing temperature. As the engine begins to operate, manifold vacuum exerts a pull on the vacuum piston to open the choke valve just enough to supply the necessary air for a running mixture.
As the engine warms, the amount the choke valve opens is governed by the pressure of air against the offset choke valve. The heat from the exhaust manifold gardually decreases the tension of the thermostat coil spring until the spring offers no further resistance to the opening of the choke valve.
With the engine partically warm, the throttle stop screw will rest on a lower step of the fast idle cam when the accelerator is released to cause the engine to idle at a slower idle speed.
When the engine reaches normal operating temperature and the accelerator is released, the throttle stop screw will rest on the lowest step of the fast idle cam, permiting the engine to return to its normal slow idle speed.
Electric choke conversion kit for the type of WW that has the stove pipe hooking up to the center of the thermostat.