Honda GCV520 Owner's Manual page 38

HELPFUL TIPS & SUGGESTIONS
The Distributor's Limited Warranty does not cover fuel system
damage or engine performance problems resulting from neglected
storage preparation.
You can extend fuel storage life by adding a fuel stabilizer that is
formulated for that purpose, or you can avoid fuel deterioration
problems by draining the fuel tank and carburetor.
ADDING A FUEL STABILIZER TO EXTEND FUEL STORAGE LIFE
When adding a fuel stabilizer, fill the fuel tank with fresh gasoline. If
only partially filled, air in the tank will promote fuel deterioration
during storage. If you keep a container of gasoline for refueling, be
sure that it contains only fresh gasoline.
1. Add fuel stabilizer following the manufacturer's instructions.
2. After adding a fuel stabilizer, run the engine outdoors for 10 minutes
to be sure that treated gasoline has replaced the untreated gasoline
in the carburetor.
3. Stop the engine, and if the fuel tank is equipped with a fuel valve,
move the fuel valve to the CLOSED or OFF position.
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