I have A 1999 62" Toro Z-master with the 27 hp Kawasaki Liquid cooled engine. When I'm Idling (not moving with the blades off) it revs up and down by itself. When I turn on the blades it goes away. It can also be choked out, meaning if I pull the choke out a little it goes away. Me and a friend who used to work on these replaced the plugs took the carb. apart and cleaned it and adjusted the valve timing. it still does it. What else can I try? It Has about 1600 hours on it. Thank you.