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Pictured from left to right are the 1955,1956,1957 vacuum advance mechanisms. These items are one of the most important parts of the ignition system as well as one of the most neglected. The timing advance after initial preset is controlled by these mechanisms. The 1957 model distributor also has a mechanical advance coupled with the vacuum advance which give a superior system. If the mechanism fails especially on the 1955 or 1956 models it results in poor performance and over heating conditions. When these units go bad they some times go undetected because the car appears to run ok and the loss of power is not detected by the little O'lady from Pasadena.
The mechanism can be checked with a timing light to insure the timing is advancing to the specifications listed in the shop manual. It can also be checked by removing the vacuum line from the carburetor, remove the distributor cap, rotate the points plate while tightly sealing the vacuum line port. If the plate moves with the port sealed the mechanism is bad and should be replaced. The point plate should occasionally be checked to insure it is not sticking which will result in the same conditions as a bad advance mechanism.