My Z210 with Captain's Call shows soot on the transom around the oval exhaust ports after each trip. Like yours, it's not heavy soot, but enough that I wash it off when returning to port (which is my garage). Like the rest of you, I wax hard once per season and the soot washes off pretty easily.

Regards,

Adjusted Choke
 

on our 7.4 weber carb, we adjusted the choke and it barely smokes at all. I don't hear much of a hissing noise, but thing runs much more smoothly after correcting the choke.