Re: 1986 Mercury 75 hp 4 cyl vs 1991 Mercury 75 hp 3 cyl.
I had a 4 cyl. 85 of the early-mid '80's. It may have been "direct charged". I just don't remember. It was as fine an engine as I ever ran. Whether it is the old cross flow design, cross flow with direct charging, or loop charged makes a significant difference in fuel consumption. I currently have a 90 triple, loop charged, that is the big brother to the '91 triple 75. It is a fuel miser, course as I recall the 85 wasn't all that bad.
I would say that since the engines are only 4 years apart, the corrosion and all considerations are really more about what kind of care did each engine receive in it's lifetime. Idle rpm for the 3 cyl is 675 +/-. Don't remember on the 85, course your engine to prop gear ratio and prop pitch will come into play too. They do make after market deflectors which are a flat plate that drops down behind the prop to slow the boat while trolling.
If you are going to do a lot of trolling then I would suggest the triple. Oil injection systems have a lean 80:1 fuel to oil ratio at idle which will help in reducing plug fouling under trolling conditions. At WOT it moves up to 50:1 as the 4 cyl used with premix.
HTH,
Mark