Honda BF9.9A Owner's Manual page 30

0 High altitude operation
At high altitude, the standard carburetor air-fuel mixture will be exces-
sively rich. Performance will decrease, and fuel consumption will
increase.
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 outboard motor at altitudes higher than
6,000 feet above sea level, have your authorized Honda Outboard
Motor dealer perform these carburetor modifications.
Even with suitable carburetor jetting, engine horsepower will decrease
approximately 3.5% for each 1,000 foot increase in altitude. The.effect
of altitude on horsepower will be greater than this if no carburetor
modification is made.
[W1
Operation of the outboard motor at an altitude lower than the
carburetor is jetted for may result in reduced performance,
overheating, and serious engine damage caused by an excessively
lean air/fuel mixture.
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