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This is the vacuum modulator that controls upshift firmness and timing. Adjusting the modulator clockwise firms and delays the upshift, while counterclockwise speeds up and softens the upshifts. Make your adjustments one turn at a time, then take a spin to evaluate the upshifts. Do not adjust the vacuum modulator any more than four turns in either direction. C4s came with two types of vacuum modulators. Early versions had a screw-in type; later models have a push-in modulator with a clamp as shown. Weak intake-manifold vacuum caused by a hot cam or torn vacuum hose will adversely affect upshifts.