The carburettor float chamber is flooding....undo the bottom nut and the chamber, complete with gasket will drop clear, revealing the white nylon float. Ideally you should take the float off by pushing out its hinge pin...this will pull the needle valve with it - which is being held open by some dirt on its seat. This can normally be done in situ, but it is easier to take the whole carb off.
John
September 2009