How to fix the throttle on my boat?

check to see if the timer base(under the flywheel) is moving smoothly,and not binding.you wont have to remove the flywheel to check it,there will be a link/rod connected to the arm that operates the throttle cam.this link/rod and timer base that it's connected to should move before the carbs even begin to open,and continue to move smoothly until it hits the total advance stop.if it is binding/sticking,you probably will have to remove the flywheel to correct it ,but you might try to spray some lube (not alot) to see if it frees up.unintended and unexpected aceleration can kill you,or even worse,get you sued or hurt.be careful.

If U mean when it comes up on plane than it should increase in speed. Try Gumout carb cleaner in the gas change the spark plugs if they have been in a long time have fresh gas and oil mix.Bottom of the boat clean.Prop not bent.