Then proceed to the second mixture screw and set it to the highest reading. Go back and repeat the process, fine tuning the idle mixture, and repeat the process for a third and final time. If your aftermarket Holley is equipped with four-corner idle circuits, set the front first, then set the rear. Recheck the front idle after the rear setting has been confirmed. Reduce the idle speed and repeat the process, perhaps several times, until the engine idles at the desired engine speed. If the carburetor has never been used previously, turn the idle mixture screws all the way in, and then back them out in unison. For the initial adjustment, all screws should be within 1/4 turn of each other, although you may have to turn the primary screws out an additional 1/8 to 1/4 turn.