Throttle Control System: Operation
When the engine is idling, intake manifold vacuum is gradually applied to dashpot diaphragm through the orifice of the dashpot check valve. This pulls up diaphragm rod so that throttle valve is held in idle position. As manifold vacuum decreases (throttle opens), vacuum in diaphragm bleeds off through check valve and rod ceases to exert pressure in closed direction. During periods of deceleration or when transmission is shifted, high manifold vacuum gradually bleeds through check valve to pull diaphragm up, allowing throttle to close slowly.
During cranking, when manifold vacuum is not present, inner dashpot diaphragm spring pressure pushes the throttle slightly open to assist in engine starting.