Small Engines (Lawn Mowers, etc.): Uneven throttle, air fuel mixture, carburator

Question
I have a Craftsman 19.5 B&S lawn tractor about 10 years old.  When I engage the blades to mow at near full throttle it seems to run fine.  With the blades up, at lower throttle speed though, it revs up, then slows down at a rate of about once per second.  What's wrong with this mower?  

Thanks.  

Answer
 Hello RC:

 The Carburator May Require Cleaning and Overhauling. Also it May Just be Out of Adjustment. Send me the Engine Model Numbers and Mower Model Numbers and I can Look at the Carburator Diagram and see if there are Adjustments. If there Are Adjustments the Air/Fuel Mixture needs to be Adjusted to Stop the Uneven Running of the Engine. Hope this Helps. Let me know what Happens, Please.

 Good Luck

 Respectfully

 John