The power valve being open will not contribute to rich idle mixtures.
If you have a rich idle mixture, your screws simply are too far out or your manifold vacuum requires the idle stop screw to be cranked in so far that you are drawing too much fuel from the transfer slots. This is a common problem.
Take the carb off and look at the primary throttle plates and look at how much of the transfer slots are exposed. Or take a pic and post it.
