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TM 9-8000
Figure 15-6. Ignition Distributor.
opened by a cam driven through the distributor shaft
across which the high-voltage surges from the coil must
from a gear on the engine camshaft. Often, the oil pump
jump after passing through the distributor.
is driven from the same gear. As the cam rotates, the
cam lobes move around under the contact arm, causing
(1)  The heat range of spark plugs is very
the contacts to open and close.
important because a plug that runs too cold will
accumulate  heavy  coatings  of  carbon  (unburned
(3) The secondary circuit through the distributor
gasoline), while a plug that runs too hot soon will burn
consists of a rotor mounted on top of the cam and a cap
up. Figure 15-7 illustrates the meaning of heat range.
with high-tension terminals. In action, the rotor turns with
When the heat path from the center electrode to the
the cam, connecting through its metal segment between
cylinder head is long, the plug will run hotter. When the
the center high-tension terminal and the various outer
path is short, the heat can escape more readily and the
high-tension terminals.  This directs the high-voltage
plug runs cooler.
surges from the coil (which enter through the center
high-tension  terminal)  to  the  various engine spark
(2)  In addition to the initial firing of the plug,
plugs (which are connected, in
there is an additional discharge of current at the spark
firing order, to the outer high-tension terminals).
plug caused by the oscillation of current in the primary
circuit between the coil winding and condenser.  This
f. Spark Plugs (Fig. 15-7). The spark plug consists
current discharge after the initial firing is not only
unnecessary for operation but also causes undue spark
of a porcelain insulator in which there is an insulated
plug deterioration and radio interference. This discharge
electrode supported by a metal shell with a grounded
of current can be shortened by using a resistor spark
electrode. Several types of spark plugs have been used.
plug. A resistor spark plug (fig. 15-8) incorporates a
They have the simple purpose of supplying a fixed gap in
resistor (about 10,000 ohms) into its insulator in series
the cylinder
with the electrode.


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