Vacuum Advance Control Solenoid: Description and Operation

Some engines have a solenoid incorporated in the distributor vacuum advance mechanism to retard the ignition timing when the throttle is closed. At a closed throttle, and with the idle adjusting screw in the closed position, electrical contacts on the carburetor throttle stop cause the distributor solenoid to energize. This retards the ignition timing to reduce emissions during hot idle conditions. Cold or part throttle starting is not penalized because the distributor solenoid is not energized unless the hot idle adjusting screw is against the throttle stop contact. Ignition timing must be set at closed throttle to give accurate setting.