1970-1979 Ski-Doo Snowmobiles Service Manual, Page 48Get this manual
TROUBLESHOOTING Compression Test Perform compression test to determine condition of piston ring sealing qualities, piston wear, and condition of head gasket seal1Remove spark plugsInsert compression gauge in one spark plug hole (Figure 5)Refer to Chapter One for suitable type of compression tester2Crank engine vigorously and record compression readingRepeat for other cylinderCompression readings should be from 120-175 psi (8.4-12.30 kgem")Maximum allowable variation between cylinders is 10 psi (0.70 kgern')ENGINE PERFORMANCE
In the following discussion, it is assumed that the engine runs, but is not operating at peak efficiencyThis will serve as starting point from which to isolate performance malfunctionThe possible causes for each malfunction are listed in logical sequence and in order of probability
Engine Will Not Idle aCarburetor incorrectly adjusted bFouled or improperly gapped spark plugs cHead gasket leakingperform compression test dFuel mixture incorrect eSpark advance mechanism not retarding fObstructed fuel pump impulse tube gCrankcase drain plugs loose or missing