Small Engines (Lawn Mowers, etc.): Toro Kawasaki Engine surging, kawasaki engine, speed circuit

Question
Hello, I have a toro z255 ztr with a kawasaki 25hp. When cold runs normal but after running mower for about 10 minutes engine suddenly begins surging under load. Disengage pto and runs a little better, engage brake, and engine runs fine just sitting. When starting to put load on engine will surge and barely runs. Sometimes will die but always starts right back up. Let sit for 30 minutes and will run fine until system heats back up and begins surging again. Lawns are getting tall!! Thanks much, Mark.

Answer
First item I would replace is the spark plugs.  We often see plugs that malfunction when warm.

Second, it sounds like the high speed circuit of the carb maybe dirty.  When you load the engine, the high speed fuel circuit is opened allowing more fuel to enter the engine.  Have you removed, dis-assembled and cleaned the carb with compressed air from an air compressor and carb cleaner?
Let me know.
Eric