2006-2009 Honda TRX250EX/TRX250X Service Manual, Page 104Get this manual
FUEL SYSTEM
HIGH ALTITUDE ADJUSTMENT
At high altitude, the standard carburetor air-fuel mixture will be too richPerformance will decrease, and fuel consumption will increaseA very rich mixture will also foul the spark plug and cause hard startingOperation at an altitude that differs from that at which this engine was certified, for extended periods of time, may increase emissions
High altitude performance can be improved by specific modifications to the carburetorIf your customer always operates the motorcycle at altitudes above 6,500 feet (2,000 meters)you should perform this carburetor modification
Even with carburetor modification, engine horsepower will decrease about 3.5 for each 1,000foOI {300-meter! increase in altitudeThe effect of altitude on horsepower will be greater than this if no carburetor modification is madeThis engine, when operated at high altitude with the carburetor modifications for high altitude use, will meet each emission standard throughout its seful lifeThe high altitude carburetor adjustment is performed as follows: Remove the carburetor (page 6-5) and float cham berReplace the standard main jet with the high altitude typeHIGH ALTITUDE MAIN JET: #92 Install the float chamber and carburetor (page 6-10)