Opens/Closes the air cleaner duct door based on vacuum input. With vacuum applied, the duct door closes. When closed, air for the carburator is taken from a space near the exhaust manifold. This warm air helps the engine when it is first started. After the engine warms up, the duct will open taking fresh cold air from the front of the car body. The duct is temperature controlled by the Air Cleaner Temperature Sensor.