Unless the idle passageway is also blocked, a power valve in either the rich or lean position won't affect the idle mixture. If the throttle blades are open far enough to expose the part throttle transfer slot, the idle mixture screw will have little or no effect. Slow the idle speed down as low as it can go and still keep the engine running, then remove the carburetor and look at the relationship of the throttle blade and the idle jet and part throttle ports. It might be necessary to drill a small hole in the throttle plate to allow the plate to close far enough for the idle needle to work properly.
Jerry
