While a tach and vacuum gauge are nice to have, untold thousands of engines have been tuned "by ear". Turn both idle mixture screws in (clockwise), gently, until they bottom out, then back them out 1 1/2 turns each. That should be a good starting point. Warm up the engine fully, choke all thre way released, and adjust the idle speed screw on the throttle linkage to get a smooth run- - - -speed isn't terribly important right now, but don't overdo it on RPM.
Now- - - -turn the idle mixture screws clockwise, one at a time, until the engine starts to stumble (too lean). Back the screw out again until you get a smooth idle. Repeat with the other screw. It might be necessary to do this several times to get things just right. Finally, adjust the idle speed to whatever curb idle suits your preference.
Once you get the engine speed a little above idle, none of the above adjustments have any effect whatsoever on fuel mileage or rich/lean condition. Cruise or high speed mixture is controlled by the main jets and power valve, and they have no adjustments other than changing jet size. If it's running way rich, check for a stuck power valve.
Jerry
