Distributor Mechanical Advance

Mechanical Advance

The mechanical advance system is fitted inside the distributor and it is possible to remove and replace the various parts. The system consists of two weights and two springs. They are below the distributor base-plate on most distributors, but above it on AC Delco distributors fitted to Vauxhall and Holden cars and Marelli distributors on Italian cars. In all forms of distributors equipped with a mechanical advance system, the part of the distributor shaft which carries the contact breaker cam is separate from the main shaft and has a small amount of free play.

One end of each weight is pivoted on a circular drive plate which moves with the distributor main shaft. The other end of each weight is attached to the breaker cam by a small spring. As the distributor main shaft rotates, these weights tend to continue in a straight line due to inertia. The pivoted ends of the weights rotate with the distributor drive plate and the weights are thrown away from the centre. This effect is called movement by centrifugal force. The force increases as the distributor shaft and drive

plate move faster. The ends of the weights apply tension to the contact breaker cam through the springs, turning the cam in relation to the mainshaft, and causing the points to open earlier in the compression stroke at higher speeds. The timing is thereby advanced. In electronic systems, the same effect is achieved by similar means. In magnetic electronic ignition systems the breaker cam is replaced by an induction cam. Advancing this induction cam advances the magnetically induced spark. In optical electronic ignition systems the breaker cam is replaced by a light interrupting blade. Advancing this blade cuts the light trigger earlier and advances the ignition spark.

With both conventional and electronic ignition systems, the weights and springs are designed to match the special requirements of the particular make and model of car. Although the mechanical advance system combines with the vacuum advance system to maintain perfect ignition timing at all engine speeds, for the purpose of inspection and repair the mechanical advance system should be regarded as operating independently.

Checking for Faulty Mechanical Advance

The mechanical advance system, like other automatic systems, depends on the distributor contact points, rotor arm, cap and shaft being in good working condition. Check and if necessary replace these before looking at the advance system. Take off the distributor cap and rotor arm. In the AC Delco distributors on Holdens , Vauxhalls and in Marrellis the weights will be immediately visible. On other distributors, you will also have to remove the contact breaker points and capacitor (see page 82), lift off the plastic sleeve at the base of the rotor shaft, and remove the distributor base-plate. The weights will then be exposed. Carry out the following checks:

1. The weights should be free to move out against the tensioning springs. All wiring should be routed clear of these weights and no other obstruction should be near them.
2. Replace the rotor arm and turn the engine by hand (and see the direction of rotation. Now gently press the cam in this direction using the rotor arm, and release it. The cam should spring back under the tension of the springs. If this does not happen, remove and clean the weights and pivotpoints, having carefully noted their position for reassembly. Replace the weights and fit new tension springs. Make sure you use the correct springs for your make, model and year of car.
3. The mechanical advance weights should not become damaged and cleaning them should be sufficient. If this is not the case the operation of the distributor must be faulty and it should be replaced completely.
4. Finally, replace the distributor components in reverse order and retime the ignition.

AC Delco distributor with the mechanical advance mechanism above the breaker plate. Marelli distributors on Italian cars use a similar design.