Spark Timing Control System: Operation

The thermal vacuum valves and the water temperature switch monitor engine coolant temperature. The vacuum delay valve (if equipped) prevents rapid vacuum changes in the vacuum advance line. The delay valve is constructed for one-way operation and when vacuum source decreases, the vacuum advance responds normally. When water temperature switch and neutral or inhibitor switch are on, the vacuum switching valve allows ambient air to enter the vacuum diaphragm and returns spark timing to normal.