I have a 22hp Kohler Command on a 60" ztr that me and 2 repair shops can't get to run right. It has 550hrs on it, but the problem started at about 425 hrs. At full throttle with no load (not mowing) you can ocassionly hear and feel a very slight roughness in the engine and it will miss a beat now and then. At full throttle while mowing in heavy grass, the roughness is more frequent and it will miss a beat, then pick back up. When it does this under load you get a loud "pop", almost like a backfire. So far the following has been done with no success. Changed plugs; cleaned out entire fuel system; replaced fuel filters; replaced stock fuel pump with an electric pump from Kohler.None of this has made any improvement. Can anyone help me get this thing running right again?