Well I finally found someone who seemed to have a little knowledge about this old Merc, an 88, not an 89 like the title says. They told me it was a link and sync problem. It's one of the straight 6's with 3 carbs, and the carbs are not all in sync with each other, one of them is opening too soon or not soon enough. I ordered a repair manual and hope to have it corrected soon.
Also on a seperate note, has anyone ever heard of a Mercury outboard running on 100:1 fuel mix? I've been told all Mercs post 1964 run 50:1, but I had a mechanic I asked about the other problem tell me that I should be running it at 100:1.