There was a recent thread where a member had exactly this issue. Wish I could find it and repost what I said. It went something like; For fuel to be leaking from the carb, both the shut off valve AND the seat in the carb must be leaking. If either were working properly, there would be no leak. I suggest replacing the float valve and repairing or replacing the shut off valve. I favour repairing iot with this kit:
http://www.mikes-afordable.com/product/A9193S.html
at $1.60, it cost next to nothing and for a few minute's work, the job is done. I have used these kits and they work VERY well.