2007-2009 Honda TRX300EX TRX300X service manual, Page 101Get this manual
FUEL SYSTEM
HIGH ALTITUDE ADJUSTMENT
At high altitude, the standard carburetor air-fuel mixture will be too rich Performance 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 emissionsHigh altitude performance can be improved by specific modifications to the carburetorIf your customer always operates the ATV at altitudes above 6,500 feet (2 ,000 meters), you should perform this carburetor modificationEven with carburetor modification, engine horsepower will decrease about 3.5 for each 1,000-foot (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 useful lifeThe high altitude carburetor adjustment is performed as follows: