1983 Ski-Doo Shop Manual, Page 208Get 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 plugability 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 inverselythe shorter the heat paththe colder the operating 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 end It 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 enginedecreasing engine speed due to misfiringreduced performanceand increased fuel consumption This is due to loss of compressionOther possible ca uses areprolonged idlingrunning the engine with the choke onor running on too rich mixture due to faulty carburetor adjustment or incorrect fuel andor fuel mixing The plug face of fouled spark plug has either dry coating of soot or an oilyglossy coating given by an excess either of oil or of oil with soot Such coatings form conductive connection between the center electrode and ground