Mercury 225 Manual page 95

1. Remove the spark plug leads. Twist the rubber boots slightly
and pull off.
2. Remove the spark plugs to inspect. Replace spark plug if
electrode is worn or the insulator is rough, cracked, broken,
or blistered, or if the precious metal is not visible on the
plug's electrode.
IMPORTANT: The color of the plug may not accurately reflect
its condition. To accurately diagnose a faulty plug, inspect the
precious metal on the plug's electrode. If no precious metal is
visible, replace the plug.
a - Precious metal
3. Set the spark plug gap to specifications.
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