2006 Troy-Bilt RZT50 with Kohler Courage 22 idles erratically, especially high RPM's and loses major power under load. Replaced the oil, filter, fuel filter, spark plugs, cleaned the air filter/sponge and used some carb cleaner to clean all the linkage and down into the carb itself. Have not taken the carb apart. None of these steps did anything. Has been running great up to the last time I mowed last week. Any suggestions are greatly appreciated.