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Overflow
Strictly speaking, the float level does not directly change your fuel mixture. That's controlled by the jet position and the piston needle If you are having an overflow issue, it might make more fuel than necessary push up through the jet into the carb body thus making you run very rich.
The un-sophisticated fix for this it to bend the float tabs so that the float closes the needle valve much sooner - you bend the float tab out a but more and the float will close the valve at about 1/2 full.
Depending on your fuel consumption this might starve the carbs for fuel on the highway, but I never had a problem with it. I found that it helped quite a bit when I was having overflow issues.
Boiling
There is one anecdotal fix for carb boiling. Install phenolic spacers between the carbs and intake. This raises the carbs 1/4-1/2 up from the intake and reduced the heat transfer to the float chambers. You can order these from Moss-Motors (the Triumph TR4a uses the same size SU HS6 carbs) They are about $5.00 each.
Incidentally, do you have the carb heat shield installed? It's critical for proper operation of the system. I'll assume you have it. You can also try putting some chrome-reflective paint on the back of shield to redirect the heat down to the exhaust. Summit or Eastwood's high-temp exhaust paint or some of that exhaust manifold wrap could help, or maybe some NASA foil insulation. In any case reducing the heat transfered to the carbs would reduce or eliminate carb fuel boiling.
Etc
A Matt said, if you suspect you are running very lean, pulling the choke will quickly answer your question by grossly enriching the mixture. I've been told (and have experienced) that SUs can run normally with an extremely over-rich mixture.
Of course, you may just have bad needle valves.
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