I'm having a problem with a Kohler 20hp. When it's running at full throttle it surges and sputters. If I back the throttle down to half it runs fine. I replaced the fuel filter, air filter, spark plugs and cleaned the carb. It still does the same thing at full throttle. I can feather the choke to about half and get it to run ok at full throttle. Has anyone had this problem? Or know what it might be?