super six intake and 318 carb

Been a while since I worked on the BBD, but if the mixture screw is all the way OUT, you have a vacuum leak or you need to plug the holes in the butterflies. Solder works well ( bonds well with brass )
Did you rebuild the carb before installing? might want to go through it just to be sure you're not trying to adjust something that has issues to begin with.
The timing being a little too advanced can also keep the idle high.