if it's the same as the '05 C50, there are two idle adjustments to make - there's a knurled knob that's very difficult to reach without removing the tank, and a throttle stop screw that takes quite a long phillips screwdriver to reach. both of these are located right at the throttle body on the clutch side of the bike, in between the jugs...don't think i have a photo, but you'll see them easily if you lift up the tank a bit.
go in TINY increments with the stop screw - like 1/16 turn at a time. it can be done with the tank properly in place and the bike running, if you're careful. you can also - once you know where it is - reach the knurled knob with the tank on in order to turn it. once you've got the idle where you want it, make sure both the stop screw and the knob are set evenly.