You have a piece of dirt stuck in the fuel inlet needle valve.
Remove the float bowl and float. Remove the needle (probably attached to the float), and clean the tip. If nothing obvious, allow some fuel to flow through the open inlet passage to flush it out.
That's probably all you need to do. If not, you will have to remove the carb completely and use carb cleaner and compressed air to disloge it.
phil_saunders@bigfoot.com
Phil Saunders
September 2008