2005-2009 Honda TRX400EX/TRX400X Service Manual, Page 103Get this manual
FUEL SYSTEM
HIGH ALTITUDE ADJUSTMENT
At high altitude, th standard carburetor air-fuel ix ture will be too rich Performa nce ill decrease, an fuel consumption will increaseA very rich ixture ill also fou the spark pl ug and cause ha rd starti ngOperat ion at an altitude th at differs fro that at which this eng ine was certified, for extended periods of timemay increase em issionsHig altitude performa nce can be improved by specif ic modifications to the carburet rIf you customer always operates the TV at alt itudes above 6,500 feet (2,000 meters)' you shou ld perform his carburetor modification Even ith carburetor modificationengine horsepower ill decrease about 3.5 for each 1,000-foot (300-meterl increase in altit udeThe effect of altitude on horsepower will be greater tha his if no carburetor mod ification is madeTh is eng ine, whe opera ted at high altitude with the carbu retor mod ificat io ns for high altitu de use, will meet each emission standard throughout its usef life The hig alt itude carburetor adjustmen is performed as fol lows :