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Be careful with the threads on the float bowl screws, they strip for fun due to being so fine and old alloy. I got Graham to helicoil all 16 of mine when I refurbed my carbs.
You could try penetrating oil but experience has taught me that how ever careful you are removing the screws, its when you put them back in the problem occurs. You jut get that very 'loose' feeling on the end of the screwdriver but by then its too late. Remove the screw and the thread comes out with it
If you don't split the carbs you can still remove the throttle slides with the needles attached to them. The needle has several grooves. If I remember correctly the standard position is 3rd groove down for the circlip. It's clearly shown in Clymer or Haynes manual.