Honda H5518H Owner's Manual page 71

10. High altitude operation
At high altitude, the standard carburetor air-fuel mixture will be too rich.
Performance will decrease, and fuel consumption will increase. Avery rich fuel
mixture may also foul the spark plugs and cause hard starting.
High altitude performance can be improved by specific modifications to the
carburetor. If you always operate your engine at altitudes above 1,800 meters
(6,000 feet), have your dealer perform this carburetor modification.
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.
1 NOTICE ]
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 1,800 meters (6,000 feet) with a modified carburetor may cause the
engine to overheat and result in serious engine damage. For use at low
altitudes, have your dealer return the carburetor to original factory
specifications.
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