Sounds like a classic case of a vacuum leak. Whether it's in the carburetor itself (i.e. the power valve, gaskets, etc), a manifold leak, or a popped-off line will take further examination. I'm willing to bet it's something simple, though. I'd change the power valve back to what it was before or just *one* click up. A faulty spring or dirty seat on the power valve could cause problems as well. I'm agreeing with Rev that an unseated power valve would cause a rich idle, but it's also possible that the spring is shot or there's a gasket not sealing off somewhere.
I believe the notion of using a 2-stage power valve as a means to have both power and economy is just hyberbole for a smoother power transition between loads (vacuum levels) upon the engine. Fuel injection made this happen with simple programming, but in this case dealing with a caburetor, it's all a matter of pressure differentials.
Good luck!
