Well, I took the butterfly off the shaft (the first butterfly screws that I have ever managed to undo without ruining them!), and found lots of wear but also a rubber seal designed to take up the air-seal slack when the shaft/housing increases. I suspect my mixture weakness problem is in fact an air leak from the manifold gasket, as another carb fitted had the same issue, and a tighter shaft! I've had the manifold machined flat (was surprisingly distorted, with inlet ducts all slightly further from the head than the exhausts), and fitted a new gasket which I am slightly suspicious of - it has a metal face and metal hole-linings (I like the hole-linings idea), and a 'normal' fibrous face. I placed the metal face against the head (with a smear of liquid gasket - wrong way around?), but I swear that there is a little 'chuff' from somewhere (so could also be an air leak to mess up the carb mixture & contribute to the light-acceleration judder some others have mentioned), so I am going to fit a fibrous gasket instead & see.
In my explorations of the carb, I also managed to snap off one of the two pressed-in slow-running tubes (doh!). I have no spares, so I soldered-up the remaining stub. I guess this helps weaken the idle mixture?
Are there any spares anywhere for these Zenith 28if carbs?
Seems that every time I fix something, I break something else!