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This photo shows both the mechanical and vacuum provisions to advance timing while the engine is running. As rpm increases, the weights, which are restrained by calibrated springs, are drawn outward by centrifugal force, advancing the engine's timing. The metal fixture on the left is a vacuum advance canister, which is connected to a vacuum source at the carburetor. As manifold vacuum increases, the diaphragm inside the canister is drawn toward the vacuum and it moves the distributor baseplate and points, also contributing to engine timing adjustment.