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In older distributors, adjusting the ignition timing is usually achieved through both mechanical advance and vacuum advance. Mechanical advance adjusts the timing based on the engine speed (rpm), using a set of hinged weights attached to the distributor shaft. These weights cause the breaker points mounting plate to slightly rotate, thereby ...
Pressure in cylinder pattern in dependence on ignition timing: (a) - misfire, (b) too soon, (c) optimal, (d) too late. In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke.
A mechanism may be provided to slightly adjust timing to allow for varying load on the engine. Since these contacts operate frequently, they are subject to wear, causing erratic ignition of the engine. More recent engines use electronic means to trigger the spark, which eliminated contact wear and allows computer control of ignition timing.
A predecessor system called "Unitized Ignition" was optional on 1972 and 1973 Pontiacs. [citation needed] Most—but not all—HEI systems have the ignition coil mounted in the distributor cap. A control module and magnetic pickup are mounted in the distributor, in place of a conventional ignition system's breaker points and condenser.
A second problem involves the mechanical cam-follower block that rides on the distributor cam and opens the points. The block wears over time, reducing how much the points open (the "point gap") and causing a corresponding change in both the ignition timing and the fraction of time during which the points are closed.
The Duraspark II ignition system is a common upgrade [1] for older Ford cars equipped with a points-type ignition. In most cases, the distributor will interchange with the older-style points distributor. The system is similar to some aftermarket systems [2] and the control module may be easily swapped. Duraspark swaps are easy and can be run by ...