1982 Ski-Doo Shop Manual, Page 273Get this manual
SECTION 04 ELECTRICAL SUB-SECTION 03 (SPARK PLUGS)
HEAT RANGE
The proper operating temperature or heat range of the spark plug is determined by the spark plug's ability to dissipate the heat generated by combustionThe longer the heat path between the electrode tip to the plug shell, the hotter the spark plug operating temperature will beand inversely, the shorter the heat path, the colder the operating temperature will beA "cold" type plug has relatively short insulator nose and transfers heat very rapidly into the cylinder headSuch plug is used in heavy duty or continuous high speed operation to avoid overheatingThe "hot" type plug has longer insulator nose and transfers heat more slowly away from its firing endIt runs hotter and burns off combustion deposits which might tend to foul the plug during prolonged idle or low speed operation
FOULING
Fouling of the spark plug is indicated by irregular running of the engine, decreasing engine speed due to misfiring, reduced performance, and increased fuel consumptionThis is due to loss of compressionOther possible causes are: prolonged idling, running the engine with the choke on, or running on too rich mixture due to faulty carburetor adjustment or incorrect fuel and or fuel mixingThe plug face of fouled spark plug has either dry coating of soot or an oily, glossy coating given by an excess either of oil or of oil with sootSuch coatings form conductive connection between the center electrode and ground