See Figure 1
This system is used on carbureted engines.
The deceleration spark advance system consists of a solenoid valve and an engine speed sensor. During vehicle deceleration, ignition timing is advanced by intake manifold vacuum acting on the distributor advance through the solenoid valve. However, when the engine speed sensor detects engine speed at or below 1,300 rpm the vacuum acting on the vacuum advance is changed from the intake manifold to carburetor ported vacuum. This is performed by the movement of the Deceleration Spark Advance System Solenoid Valve in order to maintain smooth vehicle operation.