Yamaha XS850G, Owner'S Manual

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Spark plug inspection
The spark plug is an important engine com-
ponent and is easy to inspect. The condition
of the spark plug can indicate something of
the condition of the engine.
Normally, all spark plugs from the same
engine should have the same color on the
white porcelain insulator around the center
electrode. The ideal color at this point is a
medium to light tan color for a motorcycle
that is being ridden normally. If one spark
plug shows a distinctly different color, there
could be something wrong with the engine.
For example, a very white center electrode
porcelain color could indicate an intake tract
air leak or carburetion problem for that
cylinder. Do not attempt to diagnose such
problems yourself. I n s t e a d . t a k e t h e
motorcycle to your Yamaha dealer or other
qualified mechanic.
You should periodically remove and inspect
the spark plug because heat and deposits will
cause any spark plug to slowly break down
and erode. If electrode erosion becomes ex-
cessive, or if carbon and other deposits are
excessive, you should replace the spark plug
with one of the proper type.
BP7ES (NGK) or N-7Y (CHAMPION)
Before installing any spark plug, measure the
electrode gap with a wire thickness gauge
and adjust to specifications.
When installing the plug, always clean the
gasket surface and use a new gasket. Wipe
off any grime from the threads and torque the
spark plug properly.