I have a few choices for carbs for the 1100 but I want to put aside the junk from what I can actually use so my question is what are the critical tolerances on any given su carb? How do I know when one is worn beyond its serviceable life?

Many thanks.

The main things that wear on an HS2, HS4 or HS6 SU are the throttle shaft and the body where it goes through.
Years ago SU used to sell .005"? oversized throttle shafts, you reamed the body out to fit it and Bob was yer auntie's brother.

These days they sell bushes for the body and you use a standard shaft. But you need to drill or bore the body in alignment to take the bushes. If you don't watch what you're doing you can eff the body up.

There is also a bloke in Perth? that advertised here a while ago that he could fit miniature sealed ball bearings to SUs (which is what SU should have done in the first place).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Thanks Doc, for anyone else interested I found this, it explains how to test the clearance between the piston and the vaccum chamber (and visualises how important these surfaces are in regards to corrosion/scratches/burrs)

http://www.jcna.com/library/tech/tech0006.html