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8-62
IGNITION SYSTEM
electric fuse
. . .
when
it
melts, it warns you to look for
the causes, and for damage to the engine such as
scuffed pistons, burned pistons or burned valves. In-
spect for correct spark plug heat range, overad-
vanced ignition timing, loose spark plugs, burned
head gasket, excessive detonation due to low octane
fuel and for similar causes of overheating.
Sustained Pre-ignition Damage
Sustained pre-ignition damage, (Fig. 12) usually in-
volves melting of the ceramic firing tip. Since this
requires temperatures above 1700F, it's a good possi-
bility that other components of the engine may have
been damaged by preignition. This is another sure
sign that careful inspection of the engine and its ad-
justments are required.
Chipped Insulator
Chipped insulator, (Fig. 13) usually results from
bending the center electrode during regapping of the
plug. Under certain conditions, severe detonation can
also split insulator firing ends. In a four-cycle engine,
a piece of ceramic like this is easily blown out through
the exhaust.
Mechanical Damage
Mechanical damage to the firing end, (Fig. 14) is
caused by some foreign object in the combustion
chamber. Since small objects can travel from one
cylinder to another, always check the other cylinders
to prevent reoccurrence of damage.
Inspect the distributor cap for oil film or dirt.
Pressure Type Gap Tool
Pressure type gap tools, if improperly used, im-
pose a tremendously high unit pressure on the center
electrode, (Fig. 15). This is because of compression
being exerted between the end of the center electrode
and the top of the shell. If too much force is applied
thru leverage multiplication, the center electrode seal
on any type or brand of spark plug is likely to be
damaged. There are several of these pliers type gap
tools on the market.
Reversed Coil Polarity
Reversed coil polarity can often be detected by
"dishing" of the ground electrode. Note in (Fig. 16)
that the center electrode is usually not worn badly.
This source of misfiring and rough idle can be cor-
rected by reversing the primary coil leads. An oscillo-
scope is the surest way to detect reversed polarity.
While these examples may not be conclusive in all
instances, they may indicate possible corrective pro-
cedures and further diagnosis may be necessary.
Fig. 13-Chipped
Insulator
Fig. 1 L h l e c h a n i c a l Damage
Fig. IS-Pressure
Type G a p Tool Damage
Fig.
I
&Reversed
Coil Polarity
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