Thermostatically Controlled Choke

The choke valve is located in the air horn assembly and provides a richer air/fuel mixture required for cold engine starting and operation. The choke shaft is connected by linkage to a bimetallic coil that winds up when cold and unwinds when warm.

When engine is cold, tension of the bimetallic coil holds the choke valve closed. When the engine is started, manifold vacuum is channeled through an opening at the base of the carburetor through a passage on the bottom side of the modulator diaphragm assembly, to move the diaphragm downward against the set screw. At the same time, the modulator arm contacts a tang on the choke shaft. The downward movement of the diaphragm assembly compresses the piston spring and exerts a pulling force on the modulator arm, causing the choke valve to open slightly.

The bimetallic coil is warmed by an electric heater element and, as the engine warms, heated air is routed from the exhaust crossover through a heat tube to the choke housing. The heater element and heated air entering the choke housing cause the bimetallic coil to begin unwinding and decrease the closing tension exerted against the choke valve. The coil gradually loses it tension and allows the choke valve to open.

A thermostatically controlled by-pass valve, helps prevent premature choke valve opening during an early part of engine warm-up period. The valve regulates temperature of the hot air flow to the choke housing by allowing unheated air to enter the heat tube. A thermostatic disc which is incorporated into the valve, is calibrated to close the valve at 75°F (24°C) and open the valve at 55°F (13°C).

The fast idle cam, actuated by the choke rod, contacts the fast idle speed adjustment screw and increases engine speed in proportion to choke valve opening. When the choke valve reaches the fully open position, the fast idle cam rotates free of the fast idle speed adjustment screw and allows the throttle lever to return to curb idle speed position.