Thermal Vacuum Valve: Description and Operation

Fig. 18 Idling Spark Advance System:

The vacuum chamber used in this system, Fig. 18, is of the dual diaphragm type. The main diaphragm uses ported vacuum in the conventional manner to advance ignition timing. The sub-diaphragm, however, is actuated by manifold vacuum tapped below the throttle valve and provides additional timing advance during periods of deceleration and idle. This additional timing advance results in decreased levels of hydrocarbons entering the atmosphere via the exhaust system.