High Altitude Operation - Honda HP400 Owner's Manual

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, cause hard starting and contribute to air
pollution.
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 1,800 meters (6,000 feet) above
sea level, have an anthorized Honda power carrier dealer perform this
carburetor modification.
Even with suitable carburetor jetting, engine horsepower will decrease ap-
proximately 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. A reduction in engine horsepower will reduce carrier
performance.
When the carburetor is modified for high altitude operation, the air-fuel
mixture will be too lean for low altitude use. Operation at altitudes below
1,800 meters (6,000 feet), with high altitude carburetor modifications,
may cause the engine to overheat and result in serious engine damage.
For low altitude use, reinstall the standard main fuel jet, and readjust the
pilot screw.
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