I have a Walker GHS mower with a Kohler Command 20 engine. It has about 600 hours on it. It just started to run rough, lack power, spit, sputter, and occasional light backfire/miss while running......almost acts like it is partially choked. Is this a common problem with these engines? Filters, plugs, fuels, etc. have always been maintained properly. I am going to start by replacing the gas filter, air filter, and plugs. If this doesn't help where should I look next? This Problem started after some heavy mowing which really put a load on the machine. Is there a possibility of overheating problems......possibly burnt valves??? Would appreciate any thoughts.