1972 Ski-Doo Shop Manual, Page 205Get this manual
SPARK PLUG
PLUG FACE
The plug face reveals the condition of the engine, operating condition, method of driving and fuel mixingFor this reason, it is advisable to inspect the spark plug at regular intervals, examining in particular the "plug face" i.ethe part of the plug projecting into the combustion chamberThe plug face generally reveals trouble symptoms (fig3-3-4)
SPARK PLUG CHART
The following chart indicates the various spark plugs as they apply to individual vehicles for light.and heavy duty operationBecause the P.T.Dcylinder on double cylinder engines is further away from the cooling fan it has tendencies to run hotter than the magneto side cylinder (fig3-3-6)For this reason, the chart suggests an alternative (colder) plug for the P.T.Dside cylinder when the vehicle is being used for light operation onlyFor heavy duty operation the standard plug must be installedPower take off side (P.T.O.J Magneto Side