Distributor Vacuum Control: Description and Operation

Fig. 32 Spark Control System:

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 engine coolant temperature.
Utilizing a series of BVSV (bimetal vacuum switching valves), and check valves the spark control system regulates vacuum source from port to manifold vacuum.

The distributor has 2 vacuum advance diaphragms, one diaphragm which connects to a ported manifold vacuum source to advance ignition timing after engine warm-up for maximum performance. The other diaphragm advances ignition timing during cold engine running.

Vacuum Timing Controls, California:

Vacuum Timing Controls, Federal And Canada:

The sub-diaphragm port receives manifold vacuum when coolant temperatures are below 122°F (50°C) and above 129°F (54°C).
The main diaphragm port receives ported vacuum.