is it leaking from the overflow tube? there are 3 . if its leaking from the one at the bowl that means you've got a stuck float , try tapping on the bowl with a screwdriver and see if that free's it up. Otherwise you need to take the carb apart and clean it , when you do that inspect the floats and make sure they aren't cracked ( fuel has the property to dry out plastics and they can become brittle and crack ) If they are cracked fuel can get inside and cause this problem as well. make sure the float moves freely on the pin its connected to , if it seems like its hanging up , a rebuild would be the best route to a good running carb .
Good Luck