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The ignition distributor makes and breaks the primary ignition
circuit. It also distributes high tension current to the proper spark
plug at the correct time. The distributor is driven at one half
crankshaft speed on four cycle engines. It is driven by the camshaft.
Distributor construction varies with the manufacturers, but the
standard model is made of a housing into which the distributor shaft
and centrifugal weight assembly are fitted with bearings. In most
cases, these bearings are bronze bushings.
In standard ignition, the contact set is attached to the movable
breaker plate. A vacuum advance unit attached to the distributor
housing is mounted under the breaker plate. The rotor covers the
centrifugal advance mechanism, which consists of a cam actuated by
two centrifugal weights. As the breaker cam rotates, each lobe passes
under the rubbing block, causing the breaker points to open. Since
the points are in series with the primary winding of the ignition
coil, current will pass through that circuit when the points close.
When the points open, the magnetic field in the coil collapses and a
high tension voltage is induced in the secondary windings of the coil
by the movement of the magnetic field through the secondary windings.
The design is to provide one lobe on the breaker cam for each
cylinder of the engine; i.e., a six cylinder engine will have a six
lobe cam in the distributor; and an eight cylinder engine will have
an eight lobe cam, so every revolution of the breaker came will
produce one spark for each cylinder of the engine. However, on a four
cycle engine, each cylinder fires every other revolution so the
distributor shaft must revolve at one half crankshaft speed. After
the high tension surge is produced in the ignition coil by the
opening of the breaker points, the current passes from the coil to
the center terminal of the distributor cap. From there, it passes
down to the rotor mounted on the distributor shaft and revolves with
it. The current passes along the rotor, and jumps the tiny gap to the
cap electrode under which the rotor is positioned at that instant.
This cap electrode is connected by high tension wiring to the spark
plug. As the rotor continues to rotate, it distributes current to
each of the cap terminals in turn.
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