Distributor Vacuum Control: Description and Operation

Vacuum Spark Control Valve:

PURPOSE
This valve is used in the vacuum advance circuit to provide improved driveability when the engine is cold.

OPERATION
Ports 1 and 2 and ports 3 and 4 of this valve are connected internally. When vacuum is greater at port 4 then at port 1 (such as during sudden acceleration), air must flow through the orifice to equalize pressure. This creates a momentary delay that prevents a sudden decrease in spark advance. When vacuum is greater at port 1 than at port 4, air flows freely through the unseated check valve and the pressure is instantly equalized.