If you have 4 idle screws you need to balance all of them , take them all to the bottom & back them out 1 turn then adjust them the same way turn them all out equally or in equally 1/8 of a turn each . Make sure the front & rear throttle is approx opened the same amount as well , there are idle speed screws for front & rear on all 4 bbls , the Holleys have them hidden & have to be adjusted from the bottom of the carb . If the engine still runs best with them all the way in then there is a fuel leak somewhere internally , either the powervalve is not closed at idle due to low vacuum or the wrong gaskets are installed in the carb . As mentioned often with low vacuum you have to open the throttle more to achieve Idle speed & will uncover the idle transfer slots , this makes the idle mixture screws insensitive so adjusting makes no difference , in this case you have to drill a small hole in the throttle plates [some carbs come already drilled] this allows more air into the engine without uncovering the transfer slots & retains mixture adjustment . the other thing that really helps with low vacuum engines is using smaller bleeds or restricting them this allow the lower vacuum to pull more fuel into the engine .
