Good day I have a 03 25 hp Kohler command pro. I was mowing and it just got sick. It will idle good but under advanced throttle it gets sick and will not wind up and sounds sour. The plug on the left side was black . I swapped right side plug but they are both good and the left starts to blacken. Changed head gasket and ground the valves, no sign of cracks.Still sounds the same. It can be heard loudly through the carb when air cleaner is off under full throttle. Will not come up to speed and sounds almost out of time. Cam problem? It only has 650 hrs on it. Checked ignition and tests good. I perform regular maint and use mobil 1 10w30. Any help appreciated.