One thing I might point out, that I neglected to mention earlier:
Many UPDRAFT carburetors and a very few downdraft carburetors have an idle mixture screw that is actually the final idle air bleed (the mixture screw actually meters air). In this case the mixture screw truly does alter the mixture.
Since this type of idle circuit predates the more modern circuit described previously, this is probably why the screw is labeled a mixture screw rather than a volume screw.
Jon.
