Small Engines (Lawn Mowers, etc.): John Deere 170 lawn tractor leaking carb, john deere 170, vent hole

Question
John Deere 170 lawn tractor (15+/- years old), with Mikuni carburetor that started leaking gas our of a small vent hole on the top of the carburetor.  The motor runs fine with gas spewing out of the hole. Plugging the hole makes the motor run rough and hard to start, or will not start.  Any thoughts to why this has started leaking?  Would soaking the carburetor in the "acid or whatever cleaner" from auto store help?  Or is there a "blown" part inside the carburetor?  Thanks for any suggestions,  Joe P.

Answer
I suspect the float or the inlet needle and seat are leaking causing too much fuel to enter the carb.  The extra fuel has to go somewhere so it is spilling out the vent hole.

I'd remove, dis-assemble, and clean the carburetor with compressed air from an air compressor and spray carb cleaner through all the holes.  I would also replace the inlet needle and seat and check the float.

Eric