Twin 496 Mags, starboard side has black exhaust soot on it on the transom and port side doesn't?

When I first got it, the first couple times this did not happen, the whole transom was clean when I came in. Now the starboard side is black. I have 2 fuel tanks and I fill them both at the same time. I put in 87 octane.

Any ideas as to why one side would do this and not the other?

Also, I switched from running 24 4 blade bravo's to 26 4 blade bravo's about the same time this started. But if that was the problem, I would think both motors would be doing it.

If it was me I think I would would have it checked with a scan tool and check for errors. If anything it would be a good place to start before you have to start guessing.

Thanks, I will try to have some one plug it in and check out the computer.