Honda F501 Owner's Manual page 20

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 plugs and cause hard starting.
High altitude performance can be improved by installing a smaller diameter
main fuel jet in the carburetor and readjusting the pilot screw. If you
always operate the engine at altitudes higher than 6,000 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 1,000 feet increase in altitude. The effect of altitude on
horsepower will be greater than this if no carburetor modification is made.
IMPORTANT NoT'CE 1 Once a carburetor is jetted for high altitude use,
operation at lower altitudes without rejetting may result in reduced perfor-
mance, overheating, and serious engine damage.
It is especially important to rejet a carburetor when going from a higher
altitude to a lower one. At lower altitudes, the air/fuel mixture may
become excessively lean.
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