An oversized carburetor runs lean, not rich, because the velocity of air through the too-big venturis doesn't generate enough vacuum to pull the fuel from the float chamber into the airstream. It's possible that low intake manifold vacuum might be causing the power valve to stay open most of the time, causing a too-rich mixture. It might also be prone to bog down on quick acceleration or have a flat spot just off idle because of an improper transition from idle to part throttle.
Jerry
