A rich condition can be caused by any of the items above along with the following:

Gas may be getting siphoned through the main discharge. Watch the venturi at idle. Do you see gas leaking into the bore? It should not be. This can be caused by the float bowl vent being plugged off, gas tank vent plugged, venturi gasket not fitting flat, which can hold up the venturi, or perhaps the main discharge not installed correctly. Most have a check weight installed.Take a look at this page about how the accelerator pump circuit works. There is information there about the main discharge.

Main jets might be too big. This might happen if you installed a different carburetor on the vehicle. The replacement carburetor may not be jetted for your engine.