I chased a problem like that a couple years ago and it turned out that the new foam float I got was molded a little off and it hit the body of the carb inside the float bowl so it would stick open and not shut off. So once I found it I filed the foam a little bit to cure the interference and then sealed the filed spot with clear nail polish and it's worked well ever since.

And how I found it was by removing the needle and seat from the assembled carburetor and then bending a small (~1/8") right angle hook on a paper clip and sticking it into the hole from the removed needle and seat and hooking into the tang on the float arm and gently trying to move the float up and down. When I compared how much the paperclip and float moved when the float bowl was off the carburetor to how much it moved when it was assembled (and hitting internally) it became very apparent that something was wrong there.