the float needle is not closing properly or is jammed with crud.
take the carb off the bike, take the bowl off and you will see the needle float and fuel bowl.. clean it all out with WD40 extremely thoroughly.
what is happening is the fuel float is not closing the needle properly due to poor alignment of the float or dirt wedging it open. thus fuel can still exit this when the fuel bowl is full.
about a 5 minute job
edit - whups other glen beat me to it :?
