Yamaha MM700A Manual page 81

LUBRICATION, MAINTENANCE AND TUNE-UP
Correct Spark Plug Heat Range
maximum performance and reliability. The condition of a
used spark plug can tell a trained mechanic a lot about en-
gine condition and carburetion.
Select plugs of the heat range designed for the loads and
conditions that the snowmobile will be operated. Using
spark plugs with incorrect heat ranges can cause a seized
piston, scored cylinder wall or damaged piston crown.
The standard heat range spark plug is listed in Table 8.
It may be desirable to install spark plugs of slightly differ-
ent heat range than listed to match operating conditions.
In general, use a hot plug for low speeds and low tem-
peratures, Use a cold plug for high speeds, high engine
loads and high temperatures. The plug should operate hot
enough to bum oEunwanted deposits, but not so hot that
they burn themselves or cause preignition. The insulator
of a spark plug that is the correct heat range will be a light
tan color after the engine has been operated for a while.
See Figure 35.
36). A shorter than normal plug causes hard starting, re-
duced engine performance and carbon buildup on the ex-
posed cylinder head threads. A spark plug that is longer
than normal might interfere with the piston or may cause
overheating. Physical damage to the piston or overheating
often results in permanent and excessive engine damage.
If the spark plug threads extend into the combustion
chamber (Figure 371, carbon built up on the exposed
threads may prevent the spark plug from being removed.
Forcing the spark plug out may damage the threads in the
cylinder head.
Spark Plug Removal/Cleaning
1. Grasp the spark plug lead as near the plug as possible
and pull it from the plug. If the spark plug cap is stuck to
the plug, twist the cap slightly to break it loose.
CAUTION
The dirt couldfall into the cylinder when the
plug is removed, causing serious engine
damage.
2. Use compressed air to blow away any dirt that has ac-
cumulated next to the spark plug base.
NOTE
around the base of the plug and let it soak. I f
the plug is still difficult to remove, apply
or Liquid Wrench,
penetrating oil and let it soak into the
threads again.
3. Remove the spark plug with a 14 mm spark plug
wrench.
4. Inspect the plug carefully. Look for a broken center
porcelain, excessively eroded electrodes and excessive
carbon buildup or oil fouling. See Figure 35.
Gapping and Installing the Plug
Before installing a new spark plug, carefully set the gap
between electrodes of the new spark plug. A specific gap
is necessary to ensure a reliable, consistent spark. Use a
special spark plug gapping tool to bend the ground elec-
trode and a wire feeler gauge to measure the gap between
the electrodes.
NOTE
Never try to close the sparkplug gap by tap-
ping the spark plug ground electrode on a
solid surface. This can damage the plug. Al-
ways use the special tool to bend the ground
electrode when adjusting the electrode gap.
Bending the electrode excessively may
break or weaken it.
electrode (Figure 38). The correct gap is listed in Table 8.
If the gap is correct, there will be a slight drag felt as the
wire is pulled through. If there is no drag, or the gauge will
not pass through, bend the side (ground) electrode with a
gapping tool (Figure 39) to set the proper gap.
2. Apply antiseize compound to the plug threads before
installing the spark plug.
3. Screw the spark plug in by hand until it seats. Very lit-
tle effort should be required. If force is necessary, the
threads are dirty or the plug cross-threaded. Unscrew the
plug, clean the threads and try again.
4. Use a spark plug wrench and tighten the plug an addi-
with the head. If installing an old, regapped plug and reus-
ing the old gasket, only tighten an additional 1/4 turn.
Do not overtighten. Overtightening only
squashes the gasket and destroys its sealing
ability. Compression leakage could occur
around the base of the plug. It is also impor-
tant to tighten the spark plug sufficiently to
provide a good seal. I f the plug is too loose,
hot exhaust gasses will pass around the
threads and eventually make the plug d@-
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