Honda Harmony II HRT216PDA Owner's Manual page 40

MAINTENANCE
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. 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 engine 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.
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 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 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. This is considered misuse, and the Distributor's
Limited Warranty (page 52) does not cover parts damaged by
misuse. For oxygenated fuel information refer to page 60.
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