I have a buddy with a John Deere that has a Kohler 17 HP engine (Command series) . It is 6 yrs old and has been babied the whole time and he says he has kept up with all the servicing on it. Well now he says that when it is running it is fine . But when he turns the throttle up, it knocks until the desired throttle is achieved. So when at full throttle or low or at any throttle, it is fine. But when the rpms are in transition to any another rpm setting the engine knocks. Any Ideas as where to tell him to start would be appreciated.