During my time on Enstroms (F28A and F28C) we had four in our fleet and looked after half a dozen owners' machines. I spent an extraordinary amount of time doing track and balance on them and I got the impression that some can be smoothed out and some just can't, but none of them stayed smooth for long. I certainly recognise that wunper as a particularly stubborn one to deal with. There certainly was the odd machine that was just rough and could not be smoothed out no matter what you did.