The timing of the spark is critical to the operation of the engine. For any given engine speed the correct spark timing varies according to a number of factors. They are:
The air/fuel ratio and the octane rating are not controlled by the ignition system. In older vehicles the air/fuel ratio is determined by the carburetor and in modern engines, by the fuel injection system. The fuel's octane rating is determined by its chemical blend and is set by the oil refinery.
Centrifugal advance units
The Centrifugal advance mechanism controls ignition timing in relation to engine speed. It is located within the distributor, below the contact points' base plate and operates on the distributor shaft.
The distributor shaft has two sections that turn on the same axis. The lower shaft is driven by the engine. The upper shaft controls the opening time of contact points. Both are joined by the centrifugal advance mechanism. Its function is to turn the upper shaft forward in relation to the lower shaft, proportional to engine speed.
The lower shaft has two flyweights attached, each pivoted at opposite ends and controlled by a spring. As the shaft is turned they are thrown out by the effect of centrifugal force. The faster the shaft turns the more they move out. Slowing the turning speed reduces the amount of centrifugal force, so spring force pulls them in.
A slot in each weight is keyed to the upper shaft and as the weights move out, they turn the upper shaft in relation to the lower shaft. This has the effect of advancing the opening of the contact points in relation to engine speed.
The relationship between speed and advance is determined by the tension of the springs. The maximum advance is determined by limiting the movement of the weights.
Vacuum advance units
The vacuum advance mechanism controls ignition advance in relation to engine load. Its function is to improve fuel economy and, in doing so, reduces exhaust emissions. It does not affect engine performance.
Vacuum is ported from the carburetor to a spring-loaded diaphragm housing attached to the distributor. It has a pull rod that, when operating, acts on the distributor base plate turning it opposite to the direction of distributor shaft rotation. The movement results in the contact points' opening time being advanced according to engine load.
The porting from the distributor is such that when there is a small throttle opening there is a strong vacuum applied to the diaphragm pulling the base plate and contact points to an advanced opening position. As the throttle is opened more, less vacuum is applied so the spring reduces the amount of advance until the throttle is wide open where there is no vacuum applied.