Vacuum Advance/Retard System
When coolant temperature is above 113°F (45°C), solenoid valve on retard side of distributor is activated by intake manifold vacuum. See Fig 1. Vacuum from vacuum reservoir acts on retard diaphragm, pulling breaker plate spindle and retarding ignition timing.
During cold engine operation, or when 5th gear is selected, the solenoid valve on the advance side of the distributor allows vacuum to pass. This pulls the advance diaphragm forward which moves the breaker plate to advance ignition timing.