Hmph. Have run the fual down low for the first time in a while? Sounds like maybe you have gunk in one of the Carbs. i had it with an old bike... it was a twin though so the lack of power was more obvious.
If the jets are blocked for the higher Supply then it'll stutter and splutter. Try running with the choke on slightly and see if that fixes it at all. If it does then Id say gunked up carb and Unlucky
They are a GIT!
Let us know if that does show anything... If it does ill explain what you will need to do
