Rob:
This is sort of a guess, since I haven't owned a car with a ZS carb in some while, but I do not believe that your carb has an "idle mixture screw". The ZS does have an "idle speed screw" and and "idle air screw" (sometimes called an idle air bypass). The idle speed screw will change idle RPM and nothing else. The idle air screw is useful for improving the emissions and will allow the car to run more cleanly and smooth at idle speeds. Most idle air screws on these carbs came with a plastic restrictor head that limited the rotation (adjustment) to about a half-turn. If the restrictor head has been removed, you can turn the idle air screw as much as you like. I think that the initial setting for this screw is to close it all the way (clockwise) and then unscrew it about 2 turns. In any event, the idle air screw does not affect the power and performance of the engine at speeds above about 1500 RPM.
Adjustment of the high speed fuel circuit on your car is fixed and not easily adjustable. This is the only real method that can change your engine performance (fuel-wise) for speeds above idle. "Adjustment" involves removing the dashpot piston needle and replacing it with a different one. A different discharge jet (the part that the needle fits into) is probably also avialable. To see the needle, remove your air filter and look in the throat of the carb. The needle is brass and is attached to the piston (the part that moves up and down when you rev the engine). There is also a "needle and seat" in your fuel bowl, but this serves a different function.
If your car was tuned for higher altitude, the piston needle was likely swapped for a leaner one.
Newer MGBs have these ZS carbs too, so you may want to post a note regarding tuning adjustments on the MG forum. Some of those folks may have some more recent experience with the ZS unit. The older-style SU carbs are much easier to fine-tune in my opinion. The ZS is basically a "fixed mixture" emissions-oriented carb.
By the way, in my experience, most of these cars backfire lightly during shifts. There is a device fitted called a "gulp valve" that is supposed to reduce this tendency, but it is really sort of a "band aid" solution.
G'luck.