1999 Ski-Doo Factory Shop Manual Volume Three, Page 157Get this manual
Section 04 ENGINE Subsection 08 (CARBURETOR AND FUEL PUMP)
Compensation Mode
Two conditions must be met for the compensation mode to operate: 1Engine must rev over 3000 RPM2Air temperature must exceed20C (- 4F)andor atmospheric pressure must be lower than 1000 mbarThe compensation mode brings all carburetor bowls under vacuum (lower than the atmospheric pressure) in order to make the airfuel mixture leanerThe required vacuum is produced within the needle jet air inlet
FROM AIR PUMP