1986 Yamaha V6 Special J, NOT RUNNING RIGHT!! MISFIRING AND SQEALING!!

I have a 1986 Yamaha v6 special j 220hp, and It has not been running properly since I got it. When I bought my boat about a month ago, the motor was running fine, and didn't seem to have any problems, but when I put it in and opened it up it didn't sound or feel right. First off the previous owner said he had had the boat up to 75mph, yet I could only get it up to 55. Secondly, the motor also seemed to be misfiring a slight bit. I changed the plugs, as the old ones were a bit black, and it was still misfiring, and I could feel slight jerks and fluctuation s in the motor when the boat was on plane. I decided to take off the carbs, thinking that they would be all gummed up, but they were all really clean inside. After I put the carbs back on, the motor was running worse, and it began to make a high pitch squealing noise at certain rpms, and the boat would not plane with 4 people in the boat. I'm not 100 percent sure, but a some points I don't think it was running on all six cylinders.

So currently I am rather stuck on what the issue is, I have yet to check the coils, but that's my next step. However I am convinced the problem with the lack of power, backfiring and misfiring is related to the squealing I heard. Could it be the magneto going. I have no clue.

Please let me know what you know, common issues with this motor... I don't know.

Thanks

Get a spark tester then you have a point to start off with .