Turning the mixture screw only affects the fuel and air mixture at idle. Once the engine is running above 1000 RPM or so, the idle screw could be closed off entirely, or even removed from the carburetor without a noticeable change in the mixture. The main jet and the power valve controls the mixture at any speed above idle. The mixture screw is only used to get a smooth idle, and it has no effect whatsoever on high speed mixture or gas mileage.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!