Small Engines (Lawn Mowers, etc.): B&S engine speed fluctuating, mtd riding lawnmower, internal passages

Question
I have an older MTD riding lawnmower with a 18 hp BS "Vanguard" engine.  Was running/mowing fine but when we started it the other day, the engine speed started oscilating up and down.  I can actually watch the govenor move "up and down" changing the engine speed, but can't determine why.  Changed out the fuel filter, cleaned everything up, but still the same problem.  Other than that, runs fine!  Thanks for any advice you can provide.

Answer
The engine revs up and down because of a carb problem.  When it tries to rev up it's fuel demands increase and when they can't be met, the engine dies back down.  This cycle will continue over and over and the governor is working correctly.  If the carb is equipped with an adjustable high speed mixture screw, you need to try adjusting it until the engine smooths out and then open an additional 1/8 of a turn.  If there is no adjustment or the adjustment is ineffective, you need to ensure the carb is cleaned thoroughly, including pushing a piece of tag wire through all of the internal passages and follow up with carb cleaner.  You might also want to take the numbers from the engine to a dealer and see if there is a service bulleting regarding the carburetor (I seem to recall an issue related to jet size, but it may or may not apply in your case).  Hope this helps.