ok the problem could be one of a few things,but check these in order that i write them it ok
!, as mentioned the throttle slide spring may be tipped over on way and stopping the throttle slide from opening all the way up just undo the air filter and get access to the back of the carb and run the engine and see if it lifts all the way up it should do at full revs if it doesnt then put you finger into it and push it up (without it running) to see if its jammed if it is then sort it out if its ok move on..........
2,the exhaust catylitic converter has fallen out of its pipe it sits in and its blocking the gas flow this is common so really do check it out take the exhaust off and shake it to see if you hear something loose inside if you hear something then it needs to be cut open and welded back up.
3,this engine has a 4 valve head i think (ill check tomorrow as i have a head for one)if it is indeed 4 valve if one of the rocker arms breaks it will only allow either half the amount of fuel/air mixture in or let half the amount of exhaust gas out all depending on whether it has broken an inlet or exhaust rocker arm.
either way the problem sounds like air flow to me its either the fuel /air not getting in fast enough or not getting out fast enough to achieve max revs and power.trust me im a bike doctor check all i have told you to and youll crack it.good luck with the bike and finding a job and happy xmas!
adam bike doctor(of milton keynes motorcycle centre)
December 2010