1970-1979 Ski-Doo Snowmobiles Service Manual, Page 49Get this manual
CHAPTER THREE
Smoky Exhaust and Engine Runs Rough
aCarburetor adjusted incorrectlymixture too rich bIncorrect fueloil mixture cChoke not operating properly dObstructed muffler eWater or other contaminants in fuel
Engine Loses Power
Gasoline must be of sufficiently high octane (90 or higher) to avoid "knocking" and "detonation"
FuellAir Mixture
Fuelair mixture is determined by carburetor adjustment (Tillotson) or main jet selection (Mikuni)Always adjust carburetors carefully and pay particular attention to avoiding "too lean" mixtureHeat