Honda HRS536C Owner's Manual page 40

Carburetor Modification for 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 mower at altitudes above
1,524 meters, have an authorized Honda servicing dealer perform this
carburetor modification. Even with carburetor modification, engine
horsepower will decrease about 3.5% for each 300-meter increase in
altitude. The effect of altitude on horsepower will be greater than this if
no carburetor modification is made.
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 1,524 meters with a modified carburetor may cause
the engine to overheat and result in serious engine damage. For use
at low altitudes, have an authorized Honda servicing dealer return the
carburetor to original factory specifications.
Fuel Recommendations
Use unleaded gasoline with a pump octane rating of 86 or higher. This
engine is certified to operate on unleaded gasoline. Unleaded
gasoline produces fewer engine and spark plug deposits and extends
exhaust system life. Never use stale or contaminated gasoline or an
oil/gasoline mixture. Avoid getting dirt or water in the fuel tank.
Occasionally you may hear light "spark knock" or "pinging" (metallic
rapping noise) while operating under heavy loads. This is no cause for
concern. If spark knock or pinging occurs at a steady engine speed,
under normal load, change brands of gasoline. If spark knock or
pinging persists, see an authorized Honda servicing dealer.
NOTICE
Running the engine with persistent spark knock or pinging can cause
engine damage. Running the engine with persistent spark knock or
pinging is misuse, and the Distributor's Limited Warranty does not
cover parts damaged by misuse. For oxygenated fuel information
refer to page 47.
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7. MAINTENANCE
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