When the engine is idling, the spark is timed to occur just before the piston reaches top dead center (TDC), so that the combustion can be completed by the time the piston reaches a point a little past TDC.
At higher engine speeds, there is less time for the mixture to ignite, burn and deliver its power to the piston. Consequently, at higher engine speeds, the spark must be delivered earlier in the cycle.
This is accomplished by the centrifugal advance mechanism. It is designed with two governor weights, which through centrifugal force, throw out against spring tension as engine speed increases.
This motion is transmitted to the distributor breaker point cam which is advanced in regard to the distributor drive shaft. That is, it is twisted or turned a few degrees ahead of the distributor drive shaft in the same direction of the shaft’s rotation.
This then causes the contact points to open and close earlier in the compression stroke.
Thus as engine speed increases, the spark is made to arrive in the combustion chamber farther ahead of TDC in order to give the mixture time to burn.
Centrifugal advance mechanisms
1 Depend upon engine speed to control the spark advance
2 Depend upon manifold vacuum to control spark advance
3 Help out the vacuum advance
4 Cause enormous savings in gas bills.
Even though the vacuum advance and the centrifugal advance both operate together to advance the spark, each is independent of the other.
Vacuum advance is dependent upon engine load and centrifugal advance is solely dependent upon engine speed. Therefore, 1 is correct, 2 is not, nor is 3 and 4 is irrelevant.
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Article Source : This article courtesy should goes to Auto Mechanics Autodology – Technical instruction manual by System Operation Support.