Honda EU6500is Owner's Manual page 63

Adding a Gasoline Stabilizer to Extend Fuel Storage Life
When adding a gasoline 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 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 turn the fuel valve lever to the OFF position.
STORAGE
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