Polaris 120 XC SP - Manual (Page 60)

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MAINTENANCE
Spark Plug Selection
Original equipment parts or their equivalent should always be used.
However, the heat range of spark plugs is of utmost importance. A
spark plug with a heat range which is too high will cause engine
damage. A spark plug with a heat range which is too low will cause
excessive fouling and malfunction.
In selecting a spark plug heat range for production, a manufacturer is
forced to assume that the engine is going to operated under extreme
heavy duty conditions. This protects the engine from internal damage
in the event that the purchaser actually does operate the engine in this
manner. This selection however, could cause the customer who
normally operates the engine under medium or light duty to have spark
plug failure. Refer to your Owner’s Manual Supplement for the
specific spark plug to be used in your machine.
A plug with a heat range which is too high will always cause engine
damage if the engine is operated in conditions more severe than
that for which the spark plug was intended.
CAUTION