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Black soot on transom?

Hi guys/ gals
My Dart 588 is leaving a lot of black soot on the transom every time I use the boat (22 Talon). The engine utilizes a ProSystems 1200 Dominator carb and MSD ignition. The boat runs perfectly fine on idle and thru the whole rpm spectrum. So far I have replaced both power valves in the carbs, new plugs, wires, cap rotor, coil, msd box and changed the oil that looked fine. The engine isn’t huffing any blue or black smoke or burning oil. I have also cleaned the flame arrestor very well and the carb utilizes no choke. Both sides of the transom are equally black.
This engine never left a speck of soot before, and still runs good.
Your thoughts please

Thanks
Frank

Try this, free. :)

Engine off. Carb clean w/red straw squirt down right into to while against each air bleed hole a few times. Turn your face when you do this or deal with really bad burning eyes for a minute or two. I've done it 500-1500 times, and still do it. It kills ! I'm not smart.

Should be done yearly,anyway,with every Holley or Holley based carb.

If continues to do this you can try tuning idle mixture screws clockwise another 1/8 turn. If, however, this makes the idle itself worse, than go back.

The idle mixture system does stay active through out the rpm range. I can change the gallons used of my boat significantly (small. light boat) through leaning down the idle mixture, which I do if going for an all day ride at cruise speeds.

Haha SB.... lord knows I’ve done the same thing cleaning air bleeds. Damn it burns too:bananalove:

Thanks, will give it a try.