Motorcycle Questions? Ask a Mechanic.
This is an indication of the needle valve sticking open, or being contaminated with debris of some sort.
If it is sticking open it could be due to an improperly set float or a leaking float.
If debris is the issue you'll need to remove the carburetor bowl to access the valve and remove and clean it. It may be possible to remove the bowl, remove the fuel line and then spray through the fuel inlet with carburetor cleaner to clean the needle and seat if debris is the problem.
If you let me know what carburetor is on the bike I can help you with more specifics.
I hope this helps. Thanks, P.J.