Here is another thing I ran across many moons go. I was using a fuel pump that was for a recirculating fuel system on a car that was not equipped for that. The recirculating system pump has three fittings: fuel in from tank/ fuel out to carb/ and excess fuel back to tank. This was initially used mostly on AC equipped cars to provide cool fresh fuel and defeat vapor lock. Later, all carbureted cars used this system as well as the charcoal canister vapor collector. Our first gen cars used a two fitting pump. Fuel in from tank/ fuel out to carb. Often, when someone needs a fuel pump and can't find a two fitting pump, they just use a more common three fitting pump. They just plug the return line fitting and call it a day. It works at all speeds except at idle. The extra fuel has no place to go. So it overwhelms the carb float, flooding the carb. Not by much, but enough to cause a very rich idle that can't be otherwise explained. The way I cure this is to use a three way "T" right at the fuel pump. [3/8-1/4-3/8] Then the return fuel line extra fuel goes right back to the fuel from tank line. Take a peek at your fuel pump and make sure you have the right one.