Small Engines (Lawn Mowers, etc.): riding mower dies when engaging clutch or blades, briggs and stratton, dirt dobbers

Question
I have a Murray 38" 12 hp Briggs and Stratton riding mower.  It has not been used in about 1.5 years.  The engine is running well, but dies when letting out the clutch or engaging the blades.  The blades turn freely by hand and will spin if the lever is partially engaged.  Also, the tractor begins to move when letting out the clutch.  In both cases, the engines stalls out when either the blades or clutch are fully engaged.  I have also noticed a part on the throttle assembly that is moved forward when the engine is running, but kicks back when the clutch or blades are fully engaged.  This also happens (normally I assume) when shutting off the engine when it is running at full throttle.

Answer
 Hello Walter:

 Clean and Rebuild the Carburator. The Main Fuel Jet Inside the Carburator is Clogged. Soak the Carburator Overnight(not the Plastic Carburator) to Properly Clean it. Check the Muffler and Exhaust Port for Blockage. Hornets/Dirt Dobbers Love to make Nests Inside the Mufflers of Lawn Mowers. I Suggest from your Description that the Carburator be Cleaned and Rebuilt First. Hope this Helps. Let me know what Happens, Please. Thanks.

 Good Luck

 Respectfully

 John