For a single cylinder, the 400 is about as smooth as it gets, but yes, there is a little, and I mean very little, vibration, especially at the top end. You can usually feel it in your rear end. You really should not notice it much unless you concentrate on it. If it very noticible, you might want to have the bike checked out.
I always thought the 400 was a very smooth engine until I tried the 650, which is about as smooth as engines come. It's like fine cotton compared to silk, both smooth, but ones just smoother.
You should feel only minor vibes at low revs, but enough to let you know you have a nice single cylinder thumper under you. Once under way with 5k plus rpm showing she should be nice and smooth. If you start to feel heavier synchronous vibrations at higher speeds you may have a stuck roller(s) caused by dirt and dust in the variator. Often a good blast will clear this, and it may just mean the bike has been used at lower speeds by the previous owner.
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