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