Honda Car engines GX640 Owner's Manual page 9

FUEL
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.
Gasoline is highly flammable and
explosive, and you can be burned or
seriously injured when refueling.
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Stop engine and keep heat, sparks,
and flame away.
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Refuel only outdoors.
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Wipe up spills immediately.
Fuel can damage paint and some types of plastic. Be careful not to
spill fuel when filling your fuel tank. Damage caused by spillied fuel
is not covered under warranty.
Never use stale or contaminated gasoline or an oil/fuel 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.
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