Yamaha 90 hp with strong vibration

[PRdiver]

I own a Yamaha outboard 90 hp 1995. It has an strong vibration at more than 1200 RPMs. The vibration is noticed in neutral and running forward. I replaced the spark plugs, water pump, three carburators were cleaned and syncronized, compression OK in all three cylinders but the vibration still there. Someone can tell me how to fix this problem?

[boats.net]

I'm not sure you can check for a twisted crankshaft by bringing each piston to top dead center and see it timing marks on flywheel line up.