Spark Timing Control System: Operation

The vacuum delay valve has an internal restrictor to slow air flow in one direction and a check valve which allows free air flow in opposite direction. During normal acceleration, vacuum to distributor vacuum advance mechanism is delayed by the restrictor. This results in a delayed spark advance.

Upon rapid acceleration, the pressure difference at restrictor allows check valve to open so decreasing vacuum is not delayed by restrictor. On Calif. 2.0L engines, vacuum advance is restricted by thermo valve, which reacts to changes in coolant temperature.