HONDA GCV135GCV160 - Manual (Page 24)

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The spark plug must be securely tightened. An improperly
tightened spark plug can become very hot and may damage the
engine.
Check that the spark plug washer is in good condition and
thread the spark plug in by hand to prevent cross-threading.
When installing a new spark plug, tighten 1/2 turn after the spark
plug seats to compress the washer. When reinstalling a used spark
plug, tighten 1/8
1/4 turn after the spark plug seats to compress
the washer.
After the spark plug is seated, tighten with a spark plug wrench
to compress the washer. 5.
4.