Small Engines (Lawn Mowers, etc.): RE Ride on problem, new spark, spark plug

Question
Machine starts ok but after a few minutes starts to ref up and down with carb lever moving forward and back. Eventuallly it cuts out. If I leave it a while it starts again and same happens after a few minutes. Checked usual things and changed to new spark plug but no joy.
Any ideas. Machine is a craftsman with  a brigbs&S engine 14.5HP. I tried adjusting the idle settig but made no difference

Answer
Most likely the problem lies in the carburetor/intake system.  Either a dirty carb or leaky gasket.

Does the engine have a fuel pump?
Have you replaced the air filter?
Have you replaced the spark plug?
Have you changed the oil recently?
Is you gas less than 30 dayse old?

Let me know.
Eric