1978 Ski-Doo Shop Manual, Page 268Get this manual
SECTION 05 SUB-SECTION 02, (SPARK PLUG)
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 andor 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
1978 SPARK PLUG CHART
Bombardier Limited prescribes two spark plug types for specific uses on many of its snowmobile modelsFull load: frequent use of maximum engine RPMPartial load: use of medium engine RPM