2003 F90 fuel problems

My F90 has been (is) running poorly in mid range. I took the boat to an authorized Yamaha dealer and the carbs were removed and cleaned and re-installed. Little improvement was seen by the mechanic and so the carbs were removed "again" and cleaned in an ultrasonic cleaner and re-installed. I was assured the problem was fixed and the invoice "high". The motor is still running poorly at mid range. It has 240 hours on it. I don't know what to do. The water sep. filter was changed, on board fuel filter cleaned and new plugs installed. Help please!

[boats.net]

The Yamaha dealer needs to know problem is not solved. Dirty carbs are the most common problem mid range running problems but it could be an ignition problem the dealer should be able to rule out ignition system as the problem.