Vacuum Vent Valves

Fig. 15 Typical distributor vacuum vent valve installation:

This valve, Fig. 15, is used on some vehicles equipped with a variable venturi carburetor. The primary functions of this valve are to control exhaust emissions, to prevent fuel migration to the distributor and act as a delay valve.
This valve controls exhaust emissions by delaying vacuum spark advance during light acceleration and eliminating vacuum spark advance during heavy acceleration, deceleration and idle.
When spark port vacuum is applied to the vent valve, the dump valve closes, the check valve opens and the distributor vacuum advance unit begins to evacuate through the sintered metal restrictor which acts as a spark delay valve. When spark port vacuum decreases, the check valve closes and the dump valve opens the distributor vacuum line to atmosphere. Venting the distributor vacuum line to atmosphere prevents fuel migration to the distributor diaphragm and returns the distributor to zero vacuum advance.