help with 40 yamaha

i just bought a 1999 40 yamaha today went to go for a ride but she wont open up all the way shes getting lots of compression and she starts easy just when i gives it full throtle its the same as i am in idle any ideas

You haven't given the model number, however, most were three cylinder two strokes. Look for the simple things first. All the throttle linkage there? When you open the throttle, do the linkages all move on the three carburetors and do the throttle plates open wide? There is linkage that goes up to the timing mark on the flywheel, the linkage should move the little arrow forward when you open the throttle. If all this is there and moving as it should, your problem could lie in fuel supply. If the engine has sat for any length of time, the fuel will be stale and may have gummed up some of the internal passages, within the carbs themselves. You will need to remove and overhaul them. It may also be your fuel pump. You can confirm that by pumping the primer bulb with the throttle open, if you now get lots of power then your fuel pump is not providing enough fuel to fill the carburator bowls. You will need to replace the pump. Check the Fuel filter as well.

the model # on my outboard is c40mlhx the serial # is 6h4l311120 u well it turns out i needed new plugs because my motor keeps fouling them and yes the guy i bought it of said it was a spare motor so he didn't use it much but anyways with the plugs i get usually get about 6 hours out of them without having to change them any ideas on what could be causing this sorry it took me so long to reply but my computer broke and i didn't have time to go get one until yesterday