honda-automobiles hrc216hxa User Manual

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. 
Operation at an altitude that differs from that at which this engine was 
certified, for extended periods of time, may increase emissions.

High altitude performance can be improved by specific modifications to 
the carburetor. If you always operate your lawn mower at altitudes 
above 5,000 feet (1,500 meters), have your servicing dealer perform 
this carburetor modification. This engine when operated at high 
altitude with the carburetor modifications for high altitude use will meet 
each emission standard throughout its useful life.

Even with carburetor modification, engine horsepower will decrease 
about 3.5% for each 1,000-foot (300-meter) 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 5,000 feet (1,500 meters) with a modified carburetor 
may cause the engine to overheat and result in serious engine 
damage. For use at low altitudes, have your servicing dealer return the 
carburetor to original factory specifications.