Section 06 ELECTRICAL
Sub-Section 03 (SPARK PLUGS)
03-03-3
DISASSEMBLY
First unscrew the spark plug 1 turn.
Clean the spark plug and cylinder head with pres-
surize air then completely unscrew.
WARNING : Whenever using compressed
air, always wear protective eye wear.
HEAT RANGE
The proper operating temperature or heat range
of the spark plugs is determined by the spark plug
ability to dissipate the heat generated by combus-
tion.
The longer the heat path between the electrode
tip to the plug shell, the hotter the spark plug op-
erating temperature will be – and inversely, the
shorter the heat path, the colder the operating
temperature 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.
1. Cold
2. Hot
CAUTION : Severe engine damage might
occur if a wrong heat range plug is used.
A too “hot” plug will result in overheating and pre-
ignition, etc.
A too “cold” plug will result in fouling (shorting
the spark plug) or may create 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
and / or fuel mixing. 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 conduc-
tive connection between the center electrode and
ground.
SPARK PLUG ANALYSIS
1. Overheated (light grey)
2. Normal (brownish)
3. Fouled (black)
The plug face (and piston dome) reveals the con-
dition of the engine, operating condition, method
of driving and fuel mixture. For this reason it is
advisable to inspect the spark plug at regular inter-
vals, examining the plug face (i.e. the part of the
plug projecting into the combustion chamber) and
the piston dome.
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