gstark, the manufacture of my boat chose Marine Power for the motor, I went for the 454X, carb. It also was leaving a black soot on the transom first trip out.
And a hesitation on any acelleration, at any rpm. Upon returning it to MP they said it ate a cam. Got the boat back, this time it had no pop max rpm was 4400 (800 less than original) still leaving black soot. The explanation was human error, timing set wrong. Third time out was all good, and every time since. All the communication was handled by the manufacturer so I've never spoken to MP personally. Not sure if I'm going in that direction or looking for someone new. I should tell you up front, I know less than nothing about motors, carbs, heads,etc.... I just want to go a little faster. And write the check. Repair shops are going to love me.