1967 273 4bbl with Edelbrock 500 CFM Carb

Tuning a carb with a mixture screw is always a trial and error: Usually you get the engine up to temp (Important!) set float level (on a Holley its a sight glass) get a baseline in the screws: turn them all the way in (one at a time to prevent the car from dying!) just until you feel them seat and then unscrew each 2 full turns (just to get them equalized). Reset your idle speed back to 600 (example, use whatever the FSM for your year says in gear) now turn one mixture screw 1/8 turn out, then the other - note the RPM change. If it idles faster do it again equally. If it idles faster keep going in 1/8 increments! If its slower, go the other direction. Keep track of how many your out in turns. It will soon stop making any difference and then further you go in that direction will start making the idle go down STOP! revert 1/8 turn and then RESET idle speed with idle screw to 600 (example) and start turning the screws again, left and right in the same manner. your looking for the highest idle speed (or most efficient/power) . Vacuum readings will peak here too so if your using both gauge and tach, this will help. Once the screws do nothing one way or the other idle speed wise, and the idle speed is to spec, your about as close as your gonna get short of a wideband O2 meter or a sight spark plug where you can compare combustion color to a supplied color chart. On a stock 273/4, Putting the car in gear did nothing to the RPM's on that slow old SW "S" tach, iirc it was rock solid at 600. For a factory 'hot' motor, that 273/4 was very mild at idle.