Honda Car engines GX640 Owner's Manual page 29

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 gasoline stabilizer following the manufacturer's instructions.
2. After adding a gasoline stabilizer, run the engine outdoors for
10 minutes to be sure that treated gasoline has replaced the
untreated gasoline in the carburetor.
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