Roger: what don't you like about the speedometer cable adjustment of the
mixture? Is it not fine enough? You could add a small lever arm to the
mixture screw and make the mixture adjust by push-pull with a standard vernier
type mixture knob. Depending on how long you made the lever you could push the
mixture knob in/out by a full 3" of stroke and only move the mixture screw by
1/4 turn if you needed finer adjustment.
Also, I see from the HD carb (as on the 1960s Harley Sportsters) exploded view
that the idle adjustment screw is on top of the carb. How do you get to it
with your setup? Looks tight ;o)
-Oscar