It's on my Exmark Metro. Mower has less then 100 hours on it, but is a few years old, but the other day I went to start it up and when it gets in to the top half of the throttle it sounds terrible. It was very loud (almost like a helicopter). It sounded like the muffler let loose so I check the exhaust gasket, good, so I went ahead and bougth a new muffler, still same problem. We had pereviously adjusted the valves to 4 and 6 but they were ticking so bad we re set them at 3 and 3, now the ticking is gone but its still loud. Watching the valve springs the exhaust valve isn't opening near as far as the intake. I'm thinking I might have a problem with the Exhaust valve not sealing off. Also the machine lost power now in the top end. Any advice?