I agree with Phil. SUs are different from Zenith Strombergs, more than it
looks like, and among other things normally one type carb will only use one
type jet. The position of the needle makes a difference throughout the range
and you should be able to get as rich a mix as you can use unless the needles
are WAY off. (We did have a post here a couple weeks ago where that was
apparently the case - the needles he had didn't show up in any of my manuals.)

What John said about spark plug color is correct. Too white = too lean.
Tan - correct. Fluffy black or slimy wet is too rich. Oily wet is probably
bad rings.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma