Honda F220K1 Owner's Manual page 26

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 plug 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 tiller at altitudes above 610 meters
(2,000 feet), have your servicing 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 (1,000-foot) 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 (2,000 feet) with a modified carburetor may
cause the engine to overheat and result in serious engine damage. For use
at low altitudes, have your servicing dealer return the carburetor to
original factory specification.
23