Lesson3/Learning Event 2
When the engine is running and manifold vacuum is high, atmospheric pressure pushes the
diaphragm against spring pressure and compresses the spring. The movement of the diaphragm
pulls the breaker plate to the full advance position. If the engine is then placed under load or if for
some other reason manifold vacuum decreases, the spring is able to move the diaphragm and retard
the spark.
Although the centrifugal and the vacuum mechanisms do the same thing, they cause spark advance
under different conditions. Centrifugal advance is controlled entirely by engine RPM. Vacuum
advance responds to varying loads on the engine. When the load is great, there will be little or no
vacuum advance, because the throttle valve will be open and the manifold vacuum will be low.
Most modern high-speed engines in civilian vehicles use both types of advance mechanisms on
their distributors. When both are used, the spark advance will respond to both engine speed and
load.
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