I have a 99 90hp, when I first got it I brought it in for the 10 or 50 hr. ceekup and they installed some new updates, well it started to miss so I called the factory and asked who is the best mechanic within 2 states, they told me nelson marine in white bear, I took it in and he told me the new updates were ok, but he reinstalled the old program and it has run like a watch ever since.As far as the carbon,even my new 150 yamaha, you have to do the same thing, put carbonguard in the gas and clean or change the plugs, and at the endof year I put omc engine tuner in the spark plug holes overnite let it run for 15 to20 min. the next day and it cleans all the carbon up.You have to remember this new motors demand new thinking and upkeep, because they are far better than the old motors, my 90 burns about as much gas as a old 20hp. I would call nelson marine and talk to them, there are not that many good mechanics for these new moters yet, but it’s getting better as time goes on.Hope this helps a little.