Honda GX120 Owner's Manual page 15

High altitude operation
At high altitude, the standard carburetor air-fuel mixture will be ex-
cessively rich. Performance will decrease, and fuel consumption
will increase.
High altitude performance can be improved by specific modifica-
tions to the carburetor. If you always operate the engine at
altitudes higher than 1,830 m (6,000 feet) above sea level, have
your authorized Honda dealer perform these carburetor modifi-
cations.
Even with suitable carburetor jetting, engine horsepower will
decrease approximately 3.5% for each 305 m (1,000 feet) increase
in altitude. The affect of altitude on horsepower will be greater
than this if no carburetor modification is made.
Operation of the engine at an altitude lower than the carburetor is
jetted for may result in reduced performance, overheating, and
serious engine damage caused by an excessively lean air/fuel
mixture.
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