I bought a Troy Bilt Pony some time ago with a Courage SV540. The problem was severe surging - to the point the mower was unusable. After Checking out gaskets, carbs, governors, governor springs, tank venting, etc, etc., I finally looked at the fuel shutoff valve. gas was barely dripping out - a factory defective valve. I wasted a lot of time and effort, by not checking the most basic things. Hope this helps someone.
