Vacuum Spark Advance Mechanism

The vacuum spark control mechanism can provide spark advance if a single diaphragm assembly is used or spark advance and retard if a dual diaphragm assembly is used. The diaphragm assembly used depends on the engine calibration.

Single Diaphragm Assembly:

SINGLE DIAPHRAGM ASSEMBLY
The single diaphragm assembly, Fig. 11, also changes the armature to stator relationship to give spark advance. The stator assembly position is changed by means of vacuum being applied to the diaphragm assembly. Vacuum applied to the diaphragm assembly causes the diaphragm and attached diaphragm rod to move, compressing the advance spring, which controls the rate of advance. The movement of the diaphragm rod, which is attached to the stator assembly, makes the the stator assembly move with respect to the armature. This changes the initial armature to stator relationship set during initial timing, causing spark advance. The stator assembly is mounted on the lower plate assembly which with the diaphragm assembly, is attached to the distributor base.

Dual Diaphragm Assembly:

DUAL DIAPHRAGM ASSEMBLY
The dual diaphragm assembly, Fig. 12, obtains spark advance in the same manner as the single diaphragm assembly. In this case vacuum applied to the vacuum advance port causes the advance diaphragm and attached rod to move, otherwise the action is the same. Spark retard is done by applying vacuum to the vacuum retard port. This causes the retard diaphragm to move, compressing the retard spring, which controls the rate of spark retard. Compressing the retard spring allows the diaphragm rod stop to move due to the force applied by the advance spring pushing against it by means of the diaphragm rod. The result is the diaphragm rod moves causing the attached stator assembly to change position with respect to the armature. In this instance the direction of the stator assembly movement is opposite that occurring during vacuum advance, resulting in spark retard.
NOTE: Any vacuum applied to the advance port overrides any spark retard caused by vacuum being applied to the retard port.