Timing of the spark is normally set at idle speeds by positioning the distributor body in relation to its rotating cam. The timing is set on Number One cylinder and the contacts are operated in turn by each cam lobe to provide the same timing point for succeeding cylinders in the firing order. This initial setting before T.D.C. allows time for maximum pressure in the cylinder to be developed just as the piston is descending on the power stroke.
However, as engine speed increases there is less and less time for the mixture to be ignited and for this maximum pressure to be developed. So the ignition point has to be advanced, This must occur automatically in relation to engine speed and engine load.
The speed sensitive mechanism is a centrifugal type which is located beneath the distributor base plate and rotates with the distributor cam. As engine speed rises, the flyweights on the advance mechanism are thrown outward by centrifugal force. Since the cam is able to pivot on the distributor shaft, the weights act against their springs and move the cam forward
Under these conditions, the quantity of mixture entering the cylinder is reduced and the air-fuel ratio is set for maximum economy, on a lean setting. This type of mixture burns more slowly and therefore must be ignited earlier in the cycle to derive maximum benefit from the fuel.
The load sensitive mechanism is a vacuum advance unit which is operated by intake manifold vacuum via a port on the carburettor or the fuel injection throttle body. The port is connected by a vacuum line to a sealed chamber on one side of a spring loaded diaphragm. A mechanical link on the other side, connects the diaphragm to the distributor base plate. The base plate is moveable and can be rotated, in relation to the distributor body, in the opposite direction to the distributor cam rotation.
The port in the throttle body is above the throttle plate when the engine is idling and no vacuum can reach the advance unit. However at light throttle openings, the vacuum port is exposed and manifold vacuum is transmitted to the sealed chamber. The diaphragm moves against the spring to rotate the base plate, and since the base plate carries the contact breaker, the contacts meet the distributor cam earlier in rotation, advancing the spark.
As the throttle is opened wider, there is less vacuum effect in the manifold and consequently less vacuum advance. At wide open throttle openings, there will be no vacuum advance at all and ignition advance will be determined solely by the speed sensitive mechanism.