I find vibes start around 50. They don't get worse, but neither do they go away despite revving it as high as I dare (old engine, alloy rods, .....). Usually engines have a 'period' where vibration starts and then goes away again, but mine doesen't. Shame, as I enjoy a fistful of throttle whenever possible. And so I stay under that - vibes aren't intolerable, but make you want to get back to the smooth-as-silk area.
