Jorge,
If the ball isn't being sucked flat, the problem is in the motor.
I know you've already been in the carbs, but...
I had the identical problem on my Merc Carb 200 and it turned out to be the float valve on one of the carbs. The valve seat had unscrewed a little, so the valve was shutting before the bowl was full. It would run fine at idle, but would starve two cylinders when you opened it up. To check, you should be able to turn the bowl upside down, and the float will be flush with the top of the bowl, assuming yours is the same as the carbs on the 91 through 96 Mercs.
There's another check you can do if you've got the boat in the water. When it starts to bog, hit the enrichener (choke). If it picks up more rpm, the problem is in the middle or bottom carb.
The only other thing I can think of is electrical. Some of the V6 Mercs have a dual stage stator. It uses two sets of coils, one to produce spark below 3000 rpm and the other for over 3000 rpm, if memory serves. This is what I thought was wrong with mine when I figured out it was a fuel issue.
Good luck,
Dave
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Blue Heron Boat Works
Reinventing the wheel, one spoke at a time.
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