Honda F810 Owner's Manual page 24

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 fuel mixture may also foul the spark plug and cause hard
starting.
High altitude performance can be improved by specific modifications
to the carburetor. If you always operate the tiller at altitudes higher
than 1,500 m (5,000 feet) above sea level, have an authorized Honda
tiller dealer perform this carburetor modification.
Even with carburetor modification, engine horsepower will decrease
about 3.5% for each 300 meters (1,000 foot) increase in altitude. The
effect of altitude on horsepower will be greater than this if no
carburetor modification is made.
Once a carburetor is jetted for high altitude use, operation at lower
altitudes without rejetting may result in reduced performance,
overheating, and serious engine damage.
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