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Mercury vehicles use vacuum from both the primary and secondary bores to control the spark advance. Vacuum to operate the spark advance in the primary throttle range is furnished by ports in one of the primary bores. These ports, the venturi vacuum port and the manifold vacuum port, provide a balanced vacuum. This vacuum controls the spark advance to compensate for variations in engine load and speed in the primary throttle range.

When the primary throttle plates are opened past the transition point, the secondary plates will move open. Opening the secondary plates diverts some of the air flowing through the primary bores. This reduction in the air flow will result in a lessened primary vacuum. Any lessening in carburetor vacuum will result in a spark retard. To prevent this undesired spark retard when the secondary plates are opened, vacuum is also taken from the secondary bore. This additional supply of vacuum merges with the primary vacuum to balance and stabilize the system. An accurate spark advance in both the primary and secondary throttle ranges is thus assured.

In the Lincoln applications of this carburetor,

the secondary venturi vacuum passage has been eliminated. Vacuum to control the spark advance is taken from ports in the primary bore. These ports are calibrated to provide a precise, effective spark advance in all phases of engine operation.


The spark control valve remains unchanged. It provides an efficient degree of spark advance during periods of acceleration. By providing a partially retarded spark, it minimizes detonation and increases engine responsiveness. Manifold vacuum reacting against a diaphragm operates the valve. By momentarily reducing the amount of manifold vacuum allowed to reach the distributor vacuum line, the spark valve produces an intermediate spark retard.

The spark control valve functions in a similar manner when a load is placed suddenly on the engine.


A dashpot is incorporated on Mercury vehicles fitted with automatic transmissions or power brakes. The dashpot is an anti-stall device. It



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