BOMBARDIER 1980 Shop Manual page 216

HEAT RANGE
The proper operating temperature or heat range of the
spark plug is determined by the spark plug's ability to
dissipate the heat generated by combustion.
The longer the heat path between the electrode tiP to
the plug shell, the hotter the spark plug operating temper-
ature 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 cylinder 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 firing end. It
runs hotter and burns off combustion deposits which
might tend to foul the plug during prolonged idle or low
speed operation.
M
. . CAUTION: Severe engine damage can occur if a
Y
wrong heat range plug is used:
A too "hot" plug will result in overheating and pre-igni-
tion, 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.
SECTION 04
SUB-SECTION 06 (SPARK PLUGS)
FOULING
Fouling of the spark plug is indicated by irregular running
of the engine, decreasmg engine speed due to misfiring,
reduced performance, and increased fuel consumption.
This is due to a loss of compression. Other possible causes
are: prolonged idling, running the engine with the choke
on, or running on a too rich a mixture due to a faulty
carburetor adjustment or incorrect fuel and/or fuel mix-
ing, 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 elec-
trode and ground.
SPARK PLUG ANALYSIS
Overheated
(light grey)
Normal
(brownish)
Fouled
(black)
The plug face (and piston dome) reveals the condition
of the engine, operating condition, method of driving and
fuel mixture. For this reason it is advisable to inspect
the spark plug at regular intervals, examining the plug
face (i.e. the part of the plug projecting into the combus-
tion chamber) and the piston dome .
SPARK PLUG INSTALLATION
Prior to installation make sure that contact surfaces of
the cylinder head and spark plug are free of grime,
1 . Using a wife feeler gauge, set electrode gap.
2, Apply A light coat of graphite grease over the spark
plug threads to prevent possible seizure.
3. Hand screw spark plug into cylinder head and tighten
with a torque wrench:
M (18 mm) 40 Nom (30 ft-Ibs)
W (14 mm) 27 Nom (20 ft-Ibs)
(SPARK PLUGS), PAGE 3