1979 Ski-Doo Supplementary Shop Manual, Page 136Get this manual
SECTION 05 SUB-SECTION 02 (SPARK PLUGS)
SPARK PLUG
HEAT RANGE
The proper operati ng temperature or heat range of the spark plug is determined by the spark plug's ab ility to dissipate the heat generated by combust ion The longer the heat path between the electrode tip to the plug shell, the hotter th spark plug operat in tem perature will beand inverse ly, the shorter the heat path, the colder the operating temperature ill beA "cold" type plug has relatively short in sulator nose and transfers heat very rapid ly into the cylinder head Such plug is used in heavy duty or continuous high speed operation to avoiD overheat in gThe "hot" type plug has long er in sulator nose and transfers heat more slow ly away rom its firing endIt runs hotter and burns off combust ion deposits which might tend to fou the plug during prolonged id le or low speed operation
FOULING
Fouling of th spark plug is indicated by irregular run nin of the eng in e, decreasing eng in speed due to misfirinreduced performanceand increased fuel con sumptionThis is du to loss of compressionOther possible causes are: prolonged idlin g, runn in the en gin with he hoke n, or running on too rich mix ture due to faulty carburetor adjustment or incorrect fu el andor fuel mixingThe plug face of fouled spark plug has eith er dry coat in of soot or an oily, glossy coating given by an excess either of oil or of oil with sootSuch coatings form conductive connection be11 tween th center electrode and ground