High Altitude Operation - Honda HS621 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 1,600 meters (6,000 feet) above
sea level, have an authorized Honda Snowthrower dealer perform these
carburetor modifications.
Even with suitable carburetor jetting, engine horsepower will decrease
approximately 3.5 % for each 300 meter (1,000 feet) 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 pumping 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 (8,000 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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