Running Rich means your engine is getting more fuel than air. A friend of mine had the same thing happend to his bike when he purchased it Im assuming the PO did a rejet and put in a larger main jet that was called for, we reduced the main jet by one size and problem fixed. as far as I know these carbs have mixture screws but I think that only affects idle mixture but It can help turning it down a lil, I have seen these vibrate loose in a boat and ricjened the mixture causing our boat too dround in fuel.

1986 Isuzu Trooper 2.3
2004 Suzuki GS500F