My Kohler 20hp (in a Walker) has about 1,600 hrs. It starts fine and seems to run fine when it is cold. After a while ( 20 minutes or so) the engine starts to sputter intermittently at full throttle and the rpms decrease. The sputter is similar to what you get when there is water in the fuel filter, but I have checked that several times and have also cleaned the carburetor bowl and didn't find any water. Could this be a blown head gasket. I had similar problem with a 25hp and it was a head gasket, but it had a sound that was like a sneeze when the engine was turned off and I don't hear that in this engine. Any help would be appreciated.