Where's it leaking? From the overflowhose or is the floatbowlgasket leaking? If there's feul coming from the hose it's what Fred said or the floater has a leak. If the floatbowl is leaking from where it's screwed to the carburettor you need a new floatbowlgasket.
For $20 you can buy a perfect second hand carburettor, if you're charged $20 for that little tappet you're being ripped off.