1970-1979 Ski-Doo Snowmobiles Service Manual, Page 29Get this manual
22 4PreignitionIf electrodes are melted, preignition is almost certainly the causeCheck for carburetor mounting or intake manifold leaks, also overadvanced ignition timingIt is also possible that plug of the wrong heat range (too hot) is being usedFind the cause of preignition before placing the engine back into service5Carbon fouledSoft, dry sooty deposits are evidence of incomplete combustion and can usually be attributed to rich carburetionThis condition is also sometimes caused by weak ignition, retarded timing, or low compressionSuch plug may usually be cleaned and returned to service, but the condition which causes fouling should be corrected6Gap bridgingPlugs with this condition exhibit gaps shorted out by combustion chamber deposits used between electrodesOn 2stroke engines either of the following may be the cause: aImproper fuel!oil mixture bClogged exhaust Be sure to locate and correct the cause of this spark plug conditionSuch plugs must be replaced with new ones7Wom outCorrosive gases formed by combustion and high voltage sparks have eroded the electrodesSpark plugs in this condition require more voltage to fire under hard acceleration; often more than the ignition system can supplyReplace them with new spark plugs of the same heat rangeThe spark plugs recommended by the factory are usually the most suitable for your machineIf riding conditions are mild, it may be advisable to go to spark plugs one step hotter than normalUnusually severe riding conditions may require slightly colder plugsSee Table 5CAUTION Ensure that spark plugs used have the correct thread reachA thread reach too short will cause the exposed threads in the cylinderhead to accumulate carbon, resulting in stripped cylinder head threads when the proper plug is installedA thread reach too long will cause the exposed spark plug threads to accumulate carbon resulting in stripped cylinder head threads when the plug is removed CHAPTEll TWO