30hp no power

hi, i have a 6j8 30mlhu yamaha 3 cylinder 3 carb 30hp 2 stroke. idles great, starts great, runs great.... until it gets to wide open throttle. it seems like its losing a cylinder. then when you back off the throttle it gains power for a split second then bogs again. i have had the carbs off several times and no luck or change, fuel filter, tank and lines are all clean. i bought the motor not running. needles were stuck in the seats (i know). and also installed new head gasket, impeller, and plugs. i have never been able to get it to work good at WOT. any ideas? there are adjusting screws on the carbs that are not for idle so i assumer air/fuel mix. anyone know what these should be set at? any help would be apreciated thanks.