When the engine is first started and begins to warm up, heated air from the exhaust crossover passage in the intake manifold is routed through a heat tube to the choke housing containing the thermostatic spring for regulating the choke flap. A thermostatic by-pass valve, which is integral with the choke heat tube, helps prevent premature choke valve opening during the early part of the warmup period. This is important when ambient temperatures are relatively low and adverse driveability could occur if the choke was opened too soon.
The thermostatic by-pass valve regulates the temperature of the hot airflow to the choke housing by allowing outside unheated air to enter the heat tube. A thermostatic disc in the valve is calibrated to close the valve at 75°F (24°C) and open it at 55°F (13°C).