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15-4 APPENDIX
Additional Considerations for Racing
Spark Plug Inspection
• Remove the spark plug and inspect the ceramic insulator.
*Whether or not the right temperature plug is being used
can be ascertained by noting the condition of the ceramic
insulator around the electrode. A light brown color indi-
cates the correct plug is being used. If the ceramic is
black, it indicates that the plug is firing at too Iowa tern·
perature, so the next hotter type should be used instead.
If the ceramic is white. the plug is operating at too high a
temperature and
it
should be replaced with the next colder
type.
Carbon Fouling [A]
Oil Fouling [S]
Normal Operation
[C]
Overheating [0]
CAUTION
If the spark plug is replaced with a type other than
the standard plug, make certain the replacement
plug has the same thread pitch and reach (length of
threaded portion) and the same insulator type (reg-
ular type or projected type) as the standard plug.
If the plug reach is too short, carbon
will
build up on
the plug hole threads in the cylinder head, causing
overheating and making it very difficult to insert the
correct spark plug later.
If the reach is too long, carbon will build up on the
exposed spark plug threads causing overheating,
preignition, and possibly burning a hole in the pIs-
ton top. In addition, it may
be
impossible to remove
the plug without damaging the cylinder head.
Standard Spark
Plug
Threads
Diameter:
12
mm
(0.47
in.)
Pitch:
1.25
mm
(0.049
in.)
Reach:
19 mm
(0.75
in.)
®
®
©
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NOTE
OThe heat range of the spark plug functions like ather-
@
mostat for the engine. Using the wrong type of spark
plug can make the engine run too hot (resulting in en-
gine damage) or too cold (with poor performance, mis-
firing, and stalling).
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