Small Engines (Lawn Mowers, etc.): 1992 Simplicity SunStar mower with 20hp engine, code numbers, model type

Question
I have a 1992 simplicity sunstar with a 20hp Kohler engine. Original as best as I know, with 726hr. No oil burning.
The carb was re-built and a full tune-up done in hopes of solving my problem.
THE PROBLEM
I can mow normally for about 25 mins. then without warning the mower engine revs very very fast with no control except for shutting down the throttle. There seems to be no governor control.
Is this a governor problem, if so what is the best test to ensure the diagnosis. how is it fixed.
Thank you for your attention.  

Answer
 Hello Norman:

 Send me the Model, Type and Code Numbers Off of the Engine, Please. These are Located Most Times on the Blower Housing(metal engine cover). With these I can View a Breakdown of the Engine and Governor and Better Assist you. Thanks.

 Respectfully

 John