Deceleration Valve: Description and Operation

Fig. 55 Throttle Controller Operational Diagram:

The throttle controller functions as a dashpot and a cranking opener.
The dashpot prevents the throttle valve from shutting rapidly, which helps control hydrocarbon emissions during deceleration or when gear shifting.
The throttle valve closes when vacuum in the throttle controller diaphragm overcomes the diaphragm spring. This vacuum increases gradually because of an orifice in the vacuum line.
During cranking, low manifold vacuum allows the throttle controller to push the throttle lever and provides the proper throttle valve angle to assist engine starting.