Hello. Have a Bobcat ZTR 60" w/Kohler 22hp at 500hrs. Just started cutting and made about two passes when the engine sputtered out and died, like when you get off the seat with the PTO engaged. I started it backup and I hear a horrible knocking/tapping sound. Not normal, and very obvious. The type of sound that makes you want to shut it back down right now. Raising the throttle the engine wants to die and the sound get worse. I removed the air filter and the engine got stronger and would stay running, but the noise got much worse. I tried to limp it back to the house but every time I would take either one of the control arms out of the neutral slot (same slot you have to them both in to start the engine) the engine dies. So I towed it back to the house and let it cool down. Tried again, same thing. It will idle and the sound does not have a pattern but it is consistent. Gets worse with more engine speed. Motor seems to blow a little black smoke now, but maybe I didn't notice it before. And as above if I lift an arm out of neutral it dies. Oil was low, I topped off and no change. Any ideas? Thanks for any replies.