2005 Ski-Doo Racing Handbook, Page 152Get this manual
SECTION 05ENGINE PREPARATION
Flame speed is directly proportional to piston speed in an almost linear fashionThough it is not completely understood why this relationship exists, it is thought to be related to intake speed and mixture turbulenceHence, flame speed increases as RPM increasesIt also increases as the air fuel ratio becomes leanerBecause the flame speed is slower at lower RPM's, more advance at low RPM is necessary for maximum performanceAdvancing the spark too much BTDC for the needs of the engine will cause the engine to go into detonationThe optimum ignition would then have timing significantly advanced at lower RPM, but would retard the timing at higher RPM to keep the engine out of detonationGenerally, as the ignition timing is advanced, the low end mid range power will be improved and the peak power will be moved to lower RPMRetarding the timing will generally reduce low and mid range power but may allow jetting to be leaner and increase peak powerPeak power will be moved to higher RPMThese are generalizations and ignition timing must be optimized depending on engine design, RPM range and operating conditionsIgnition advance on Rotax engines is measured by linear distance of piston travel BTDCA dimension taken through straight spark plug hole in the center of the head is direct measurementA dimension through an angled plug hole on one side of the head is an indirect measurementA direct measurement can be converted to degrees of crankshaft rotation by the appropriate formulasInitial ignition timing procedures can be found in the Shop Manual for the particular model being worked on