Merc 700 black exhaust ?

[SeaHorse]

My 79 Merc 700 3 cylinder runs fine. The carbs must be set rich at idle else it dies when I accelerate. The problem is my white boat develops a black band of gook, kind of like a 10" bathtub ring, across the back. I only operate at low speeds when putting in or out for about 300 yards, the rest of the time I go about 25. I run only Pennzoil semi-synth blend oil. Can I eliminate the goo with pure synthetic oil?

 

[The Marine Doctor]

Re: Merc 700 black exhaust ?

I do not believe that changing oils will help.<br /><br />If you are getting the black line on the boat, there is definately too much fuel going through your engine.<br /><br />Now if you have to run the carbs that rich, I suspect there is something more wrong with your engine other than a rich condition.<br /><br />The 70hp Merc's will run with a broken reed pedal, but require rich carbs to do so.<br />You might also want to check the restrictors under the intake ports to see if they are damaged or gone.<br /><br />TMD