Small Engines (Lawn Mowers, etc.): craftsman 12 hp riding lawn mower, briggs and straton, craftsman riding lawn mower

Question
I have a 5 speed 38" craftsman riding lawn mower.  It has a 12 HP 4 cycle Briggs and Straton motor.  The problem with it is that gas leaks from the carb and gas reserve when the mower is sitting and turned off.  Also, when the blades are engaged, the mower turns off.  I recently changed the air filter and sharpened the blades.  I replaced the old gas and oil.  What could be wrong?  

Answer
Hi Mike,

The leaking is most likely due to a carb float problem.  Either the float is fuel logged and heavy or the floats' valve and seat are worn or have a piece of debris stuck between them.  
The mower shuts off when the blades are engeged due to a safety switch that is faulty or needs adjustmment.  Switches are used for the seat and sometimes with the engagement system.

Let me know how you make out.
Michael