1985 Ski-Doo snowmobile Service Manual, Page 351Get this manual
Section 04 ELECTRICAL SYSTEM Sub-section 03 (SPARK PLUGS)
DISASSEMBL Y
First unscrew the spark plug one turn Clean the spark plug and cylinder head With pressurize air then completely unscrew
FOULING
Fouling of the spark plug is indicated bv Irregular running of the engine, decreased engine speed due to misfiring, reduced performance, and increased fuel consumption ThiS IS due to loss of compression Other 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 andor fuel mlxlrlg The plug face of fouled spark plug has elthel 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
HEAT RANGE
The proper operating temperature or heat range of the spark plugs is determined by the spark plug's ability to dissipate the heat generated by combustion The 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 operatlrlg temperature will be "cold" type plug has relatively short insulator nose and transfers heat very rapidly into the cylinder head Such plug is used In heavy duty or continuous high speed operation to avoid overheating The "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