2003 Polaris Deep Snow Snowmobiles Service Manual, Page 45Get this manual
MAINTENANCE Spark Plug Selection
Original equipment parts or their equivalent should always be usedHowever, the heat range of spark plugs is of utmost importanceA spark plug with heat range which is too high will cause engine damageA spark plug with heat range which is too low will cause excessive foulingIn selecting spark plug heat range for production, manufacturer is forced to assume that the engine is going to be operated under extreme heavy duty conditionsThis protects the engine from internal damage in the event that the purchaser actually does operate the engine in this mannerThis selection however, could cause the customer who normally operates the engine under medium or light duty to experience spark plug failureCAUTION: plug with heat range which is too high may cause engine damage if the engine is operated in conditions more severe than that for which the spark plug was intendedA new engine can cause temporary spark plug fouling even though the heat range is correct, due to the preservative which has been added during assembly of the engine to combat rust and corrosionAvoid prolonged idle speeds, as plug fouling and carbonization will resultAlways use resistor type spark plugsNOTE: Incorrect fuel mixture can often cause spark plug to appear to be too dark or too light in colorBefore changing spark plug heat ranges, be sure the correct jetting is installed in the carburetors and proper carburetor function is verifiedThe spark plug and its condition is indicative of engine operationThe spark plug firing end condition should be read after the engine is warmed up and the vehicle is driven at higher speedsImmediately check the spark plug for correct color