Section07ELECTRICALSYSTEM
Subsection 01 (IGNITION SYSTEM)
The longer the heat path between the electrode
tip to the plug shell, the hotter the spark plug oper-
ating temperature will be and inversely, the short-
er the heat path, the colder the operating temper-
ature will be.
A “cold” type plug has a relatively short insulator
nose and transfers heat very rapidly into the cylin-
der head.
Such a plug is used in heavy duty or continuous
high speed operation to avoid overheating.
The “hot” type plug has a longer insulator nose
and transfers heat more slowly away from its fir-
ing end. It runs hotter and burns off combustion
deposits which might tend to foul the plug during
prolonged idle or low speed operation.
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12
1. Cold
2. Hot
CAUTION: Severe engine damage might occur
if a wrong heat range plug is used.
Atoo“hot” plug will result in overheating and pre-
ignition, etc.
Atoo“cold' plug will result in fouling or may cre-
ate carbon build up which can heat up red-hot and
cause pre-ignition or detonation.
Fouling
Fouling of the spark plug is indicated by irregular
running of the engine, decreased engine speed
due to misfiring, reduced performance, and in-
creased fuel consumption. This is due to a loss
of compression. Other possible causes are: pro-
longed idling, or running on a too rich mixture due
to a faulty carburetor adjustment or incorrect fuel.
The plug face of a fouled spark plug has either a
dry coating of soot or an oily, glossy coating given
by an excess either of oil or of oil with soot. Such
coatings form a conductive connection between
the center electrode and ground.
Spark Plug Analysis
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123
1. Overheated (light grey)
2. Normal (brownish)
3. Fouled (black)
The plug face (and piston dome) reveals the condi-
tion of the engine, operating condition, method of
driving and fuel mixture. For this reason it is advis-
able to inspect the spark plug at regular intervals,
examining the plug face (i.e. the part of the plug
projecting into the combustion chamber) and the
piston dome.
Spark Plug Installation
Prior to installation make sure that contact sur-
faces of the cylinder head and spark plug are free
of grime.
1. Using a wire feeler gauge, set electrode gap
according to the following chart.
2. Apply anti-seize lubricant over the spark plug
threads to prevent possible seizure.
3. Hand screw spark plug into cylinder head and
tighten with a torque wrench and a proper
socket.
162 smr2005-057
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