Check that the, slide, jets and needle position are the same in both carbs, and if they are not - set the one on the side with the sooty plug to be the same as the other. I will guarantee you have not got that problem. More likely to be flooding due to crap under the float needle, or a sinking float. Even if the idle/low range circuit is set dramatically wrong, to get a plug looking like that is unusual. If you've got flexible mounts, it is possible that one carb is hitting something while bouncing around causing the float needle to be jolted off its seat.