I have had excellent results with Spectro XL-1 motorcycle wash. It also works well on removing brake dust from aluminum wheels.
Ken

When we wash our boats it goes directly into our brackish lake, so I use Simple Green. It is biodegradeable and it seems to work fairly well. It does work well on the rest of the boat. Need to do a little scrubbing, but I can still see a slight stain on the back edge of the swim platform. I will probably have to buff it out with a lite compund type cleaner/polish.

You would be supprised how well Method wipes work.
(I pick them up at office depot- believe it or not)
I tried them one day when I was in the shop goofing off.
My wife threw them out, so I dug them out of the trash can and figured I could use them for something in the shop.
Can't believe how good it works.

Running too rich and you have black soot on the transom.

Run too lean and you burn holes in your pistons.

The former is better than the later.

Lean out the carb just a smidgeon.

# 50 cleaner wax will get the soot off.

Try 3-M Black Streak Remover or Mequiar's Black Streak Remover. Spray on and wipe; will take it right off

Use the 3M - I'm a shareholder

The Works tub and tile cleaner. You can get it at the Dollar Stores.

I have used West Marine hull cleaner which is some strong stuff it seems to take some off but not all. Is it possible for the soot to stain and never be removed?

Just joined the forum,have been reading for a while and finally joined so I can play while working.I have 525s in my Hallet 300 and the starbord motor soots the transom as well as dirties the oil more than port but the port motor does not.I have only run 91 octane in it and thought that was the problem but they are not both doing it.Sounds like I may have a bad O2 sensor also,both run great with no problems.Any input??I only use Amsoil Dominator oil in these.