Honda Power Products HSS970 Manuel De L'utilisateur page 46

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.
High altitude performance can be improved by specific modifications
to the carburetor. If you always operate your snowblower at altitudes
above 1,500 meters (5,000 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.
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 1,500 meters (5,000 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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