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In Bruce Bingham''s book "The Sailor''s Sketchbook" he has a page refering to a "magical" cleaning solvent for sailors. It ends up being brush cleaner which you can buy at any hardware store for a reasonable price. I have been using it for two seasons to clean the diesel residue around my exhaust port (like yours), as well as the brown waterline buildup that seems to resist everything else but "ON&OFF", which is too acidic for my liking.

Just remember to rinse with clean water when you are done.