75-77 Distributor Vacuum Advance - L-48 Except California

The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel ecomony and throttle response. Once the engine load increases, this "over-advance" condition must be eliminated to produce peak power and to eliminate the possibility of detonation ("engine knock").