With The fuel we get today it takes just a little bit rich to get that light soot coating on the plugs. Check your heat risor valve maybe stuck open. That would get less heat to the end cyls. & cause it to be rich. Check the float level in the carb. Try leaning down the idle mixture screw. Make sure the vacuum outlet in the bottom of the carb has a passage to the intake manifold vacuum. You may have to go 1 or 2 thousands smaller on the main metering jet.
