ORIGINAL: Richard L.
This is surprising to hear because all five of my YS 110's are hard mounted, and all five idle as smooth as silk. Is your prop balanced?
Here is one example of a rock solid idle at 1400-1500 rpm (no vibration): [link=http://www.warbirdobsession.com/h9_corsair/videos/SlowIdle.wmv]YS 110FZ Idle[/link] - WMV, 25 seconds
what are you using for a muffler?