One of the four carburettors has a float needle valve fault. Each carb should have a rubber pipe connected to the base of the float chamber, so first you have to find out which carb is at fault. then, its carb off time, float chamber removed, ready for a clean up. You should see a screw at the bottom of each float chamber. This allows the fuel to be drained off, together with any grot that may be present.
John
April 2009