This thread made for some fascinating Saturday morning reading. =)
Yuki
I had a yeti with CRAZY vibe. NOTHING fixed it. After tuning its dead smooth 
explain that!
Underpants gnomes did it.
Yuki
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I’m more interested in YOU explaining that. 
What kind of tuning did you do? Just moving the axle?
Yetis are also plastic with spacers… that’s another layer of movement that’s possible in a way that all-metal yoyos can’t achieve… the topic title refers to metal.
Teflon taping and screwing/unscrewing
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I currently have a theory that it’s because no axle is straight, and always off a varying amount. Adding tape into the mixture allows you to re-balance the axle with something that can easily be manipulated
In the past I have gone as far as chasing the threads in the halves with a bottom tap, running the axle through a die, measuring the axle/subtracting the width of the bearing and dividing the remainder by two then putting loctite on the axle and screwing it in exactly half of the remainder. Then trying a half dozen different bearings until I find the one that that particular yoyo likes best. So yes, in my opinion yoyos can be improved upon or “tuned”.
Another reason for me to get some taps. I learned how to use them in a college class.
My way of doing that was to roll small wads of paper and stuff them in the hole to the proper depth, then just put the axle in as far as the wads.
tuning wont work with a YYJ Lyn Fury 
I don’t think anyone ever said it would. The “adjustable gap” feature gives a wide range of positions.
I’ve done this, too. Strikes me as a very sensible way to create a depth stop!
It didn’t yield any results, mind you.
Nope. Never expected any. It was mainly done just to keep enough threads on both halves for good tightening.
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Exactly why I did it, too. I did it on a Token, a yoyo already known as an easy stripper, and it has a very short axle. Might as well maximize threads per side and try to keep it in good shape!
I hated that about the token. I never unscrewed mine. Maybe once or twice to switch out a bearing.
Here is how I see it.
The pic is a bit exaggerated, but basically I think that’s the reason why some metal throws are tunable. In real world, the bearing seat and post might be only slightly off, but the whole yoyo could be totally unbalanced. This happened with my stripped supernova (you know yoyofactory are notorious for their tight bearing seat), the previous owner simply forced a bigger axle to fit, it worked but it’s “locked” in one side and not too straight, it still has vibe since the axle cannot “wiggle”, therefore it’s not only holding the yoyo sides together but also hold it at certain angle.
Long story short I stripped that again and this time I had to drill the axle hole through and put longer axle, now it doesn’t vibe.
This also happens on a cheap chinese yoyo I had, since the bearing seat (or post? the one that hold the bearing) is loose, I put some clear paint over the seat (or post) so the bearing fits snugly, it reduced vibe pretty dramatically.
The best technique I found for tuning yoyos was to mess with a variety of bearings.
Bearings differ quite a bit both in their outer and inner diameters. Some bearings are tighter while others are looser.
This made a huge difference in vibe. It was also a reliable and repeatable technique which didn’t require me to worry about a perfectly tightened yoyo. I ended up with a very wide assortment of bearings and just swapped them until I found something that worked.
That said, I had little luck with the ‘aligning’ halves strategies.
I think you guys are onto something with the bearing tightness.
On the other hand, some yoyos feel downright “loose” in the bearing seat (the bearing will fall off if you invert the half) and still end up being smooth.
It’s all a mystery, I say. But trying different bearings is sure to help some throws. I definitely notice a difference between bearings in my new Flow. They’re ALL super-smooth, but a 10-ball gets it into “are you kidding me?” smoothness.
There’s a definite correlation between a loose bearing seat and vibe. I’ve reproduced this many times.
I’ve actually found that some of the cheap, generic concave bearings have a slightly larger outer diameter than other bearings… those are my ‘ace in the hole’ bearings for a vibey throw.
Jason