Glenn,

The power valve is a doo-dad inside the carburetor that gives it a little extra fuel when vacuum is low. This would be when you are climbing a hill or have your foot in it, so to speak.

The base gasket has holes that be aligned under a little passage in the base of the carburetor...sometimes the phenolic spacer has a small notch too. The passage inside the carburetor must be open to the inside of the manifold. I think checking the float level would be good insurance too.

Those are a few of the things that could cause a rich mixture. Maybe someone will be along with more things to check out. Good luck.

Stuart