Colt L4-1597cc 1.6L VIN 3 2-BBL (1983)
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.