i am experiencing some dirty exhaust fume stains on my transom. The engine of my E-38 is running fine. Fact is I just finished a 600 mile trip up to Lake Huron and back to my home port of Sandusky, Ohio where I motored for about 60 hours without incident. My engine is a universal M-40 with a primary Racor water separator 20 micron element and a 2 micron secondary fuel filter system. Lately I have noticed a bit of sludge in the sediment bowl of the Racor filter when I change the element ( every 50 hours or so). I do use an additive every time I fill up. My boat is a 1985. I have owned it for 6 years so I dont know if the previous owner had any problen with dirty fuel before . I dont know how much sediment is normal or if I even have a dirty fuel issue since the boat runs great However, I am thinking I should do one of three things to insure that I do have clean fuel: pump out all the fuel in the tank and clean the tank , or polishing the fuel by putting on a internal fuel polishing system, or polishing the fuel thru an external system. All alternatives are expensive. I need your help and advice as to how to proceed .