My 23hp Kohler surges when ever it doesn't have a load on it. When I just drive from one place to another it surges, as soon as I load it by engaging the mower deck it stops. Changed fuel filter and gas cap and it still does it. Recently I bought non ethanol gas filled up the tank cut half the yard and it started surging. Ran the mower out of gas put new ethanol gas and it continues to run the same way. Anyone have an idea what could've happened?