Vacuum Advance

Vacuum Advance:

The vacuum advance controls affect the time at which each spark plug fires. The ignition timing control system changes the ignition timing in response to manifold vacuum and coolant temperature.
The distributor has 2 vacuum advance diaphragms, 1 diaphragm which connects to manifold vacuum to advance ignition timing during and after engine warm-up for maximum performance. The 2nd diaphragm advances ignition timing based upon engine temperature.
During cold engine start, manifold vacuum is supplied to vacuum advance diaphragm A, through a check valve, and the thermovalve. When engine coolant temperature reaches 140°F (60°C) the thermovalve releases vacuum from diaphragm A by opening a passage to atmospheric vent located on the air cleaner.
Prior to engine coolant reaching 140°F (60°C) both vacuum advance diaphragms receive manifold vacuum signal from the intake manifold, through the thermovalve. After engine warm-up (coolant above 140°F (60°C)) the thermovalve opens a passage to atmospheric vent in the air cleaner releasing vacuum in diaphragm A. Diaphragm B which also receives a vacuum signal from the thermovalve retains vacuum to increase engine performance during normal driving.