The float valve may need to be replaced. Like a leaky faucet if the seat becomes cracked or otherwise damaged the valve won't shut off completely.
A common mistake many people make is trying to clean the float valve in carburetor cleaner. The needle and/or seat will have rubber sealing surfaces that the carburetor cleaner can damage.
The sealing surfaces will also dry out and become brittle over time.
You should also check the float level. If it is too high it might not be able to shut off the flow. Any time you remove the float valve the float height must be checked / adjusted. It is a good idea to check it every time you have the carburetor apart anyway.
Rod