Honda UMK425T Owner's Manual page 42

Carburetor Modification for High Altitude Operation.
At high altitude, the standard carburetor air-fuel mixture will be too
rich. Performance will decrease, and fuel consumption will increase. A
very rich mixture will also foul the spark plugs and cause hard starting.
Operation 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 carburetor. If you always operate your grass/weed trimmer at
altitudes above 610 meters, have your dealer perform this carburetor
modification. This engine, when operated at high altitude with the
carburetor modifications for high altitude use, will meet each emission
standard throughout its useful life.
Even with carburetor modification, engine horsepower will decrease
about 3.5% for each 300-meter increase in altitude. The effect of
altitude on horsepower will be greater than this if no carburetor
modification is made.
CAUTION:
When the carburetor has been modified for high altitude operation, the
air-fuel mixture will be too lean for low altitude use. Operation at
altitudes below 610 meters with a modified carburetor may cause
the engine to overheat and result is serious engine damage. For use at
low altitudes, have your servicing dealer return the carburetor to
original factory specifications.
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