That's a common problem with all carbs. once you get 1.5 turns out on a mixture screw that's at the approximate limit that that circuit will carry, rpm is down, so you crank on the throttle setting and get you extra fuel. Once you give the engine the required fuel for good idle he won't need the extra throttle opening for rpm. In my experience with the six packs 5/8 out on the outboards and that will typically give you enough adjustability on the center, again up to 1.5-1.75 turns out. If the engine is still climbing in rpm, you may need to go more than 5/8 on the outboards. my