Small Engines (Lawn Mowers, etc.): 18 hp B&S surging - seems to be governor, murray lawn tractor, briggs and stratton

Question
Hello Eric.
I'm working on an 18 horse Briggs and Stratton in a Murray lawn tractor.  
When the engine is off, the throttle plate does what it should (open when full throttle, closed at idle position).  When the engine is running, the engine will die at idle throttle position, and at anything more than idle throttle position, the engine surges. While running (barely running) at full throttle position, the throttle plate is closed most of the time.  About once a second it opens partially, fuel is sucked in through the jet and the engine speeds up, followed by an immediate closing of the throttle plate and the engine speed slowing again.  As a test I hooked the throttle cable directly to the throttle plate mechanism/linkage, bypassing the spring governor assembly.  The engine ran perfectly (no surging).  But I would prefer to have the governor working properly, so I don't have to be constantly adjusting the throttle.  I just don't know even what to look for as a fault on that seemingly bizarre contraption (the governor assembly) bolted to the engine.  Any thoughts on this would be greatly appreciated.
-Brett

Answer
Most of these surging problems are actually carburetor problems and not governor problems.

Have you removed, dis-assembled and cleaned the carb with compressed air from an air compressor and carburetor cleaner?
How old is the mower?  Has the carburetor ever been serviced?
Let me know.
Eric