Spark Timing Control: Operation

The spark delay valve has an internal restrictor to slow air flow in one direction and a check valve which allows free air flow in the opposite direction. During normal acceleration, spark port vacuum to distributor vacuum advance is delayed by the restrictor. This results in a delayed spark advance. Upon rapid acceleration, the pressure difference at the restrictor allows the check valve to open so the decreasing port vacuum is not delayed by restrictor. On California vehicles, thermo valve restricts vacuum advance until engine warm up is accomplished. This helps speed warm up of catalytic converter.