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Additional Considerations for Racing
Spark Plug:
The spark plug ignites the fuel and air mixture in the com-
bustion chamber. To do this effectively and at the proper
time, the correct spark plug must be used, and the spark
plug must be kept clean and the gap adjusted.
Tests have shown the plug listed in the "General Informa-
tion" chapter to be the best plug for general use.
Since spark plug requirements change with the ignition
and carburetion adjustments and with riding conditions,
whether or not a spark plug of the correct heat range is
used should be determined by removing and inspecting
the plug.
Terminal [Aj
Insulator [SI
Cement IC]
Gasket
[D]
Center Electrode [E]
Gap [F)
Reach [G]
Side Electrode [H]
When a plug of the correct heat range is being used,
the electrodes will stay hot enough to keep all the carbon
burned off, but cool enough to keep from damaging the en-
gine and the plug itself. This temperature is about 400 -
800"C (750 -1450°F) and can be judged by noting the con-
dition and color of the ceramic insulator around the center
electrode. If the ceramic is clean and of a light brown color,
the plug is operating at the right temperature.
A spark plug for higher operating temperatures is used for
racing. Such a plug is designed for better cooling efficiency
so that it will not overheat and thus is often called a "colder"
plug. If a spark plug with too high a heat range is used - that
is, a "cold" plug that cools itself too well - the plug will stay
too cool to bum off the carbon, and the carbon will collect
on the electrodes and the ceramic insulator.
The carbon on the electrodes conducts electricity, and
can short the center electrode to ground by either coating
the ceramic insulator or bridging across the gap. Such a
short will prevent an effective spark. Carbon build-up on the
plug can also cause other troubles. It can heat up red-hot
and cause preignition and knocking, which may eventually
burn a hole in the top of the piston.
®
APPENDIX 15-3

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