If your Holley carburetor is running rich at idle and low speeds, for no apparent reason, check the front face of the main body. It is probably dished and needs to be machined flat. It can be done with a file, flat plate and spotting blue as well. I've found this problem on several Holleys lately. The warpage gets to the point that the gasket can't take up the clearance and the gas runs across the face rather than following the passages.