Distributor Vacuum Vent Valve: Operation

When spark port vacuum is applied to the vent valve diaphragm, the dump valve closes, the check valve opens and the distributor diaphragm begins to evacuate through the centered metal restrictor. At the same time, any fuel in spark port line is evacuated. The centered metal restrictor located in the vent valve diaphragm acts as a spark delay valve slowing spark advance.

When spark port vacuum decreases, the check valve closes and the dump valve opens the distributor vacuum line to atmosphere. Venting the distributor line prevents fuel migration to the distributor diaphragm and returns the distributor to zero vacuum advance.