First, what kind of moped do you have? That makes a huge difference. If your iddle is really fast, you might try tuning your throttle. On a Dellorto carburator, there's a screw on the left side that can adjust the throttle. My Bianchi also has a small screw-like thing where the throttle cable goes into the handlegrip that can be turned also. Either or both of those shoul be used.
Of course, your moped might have a different system. Again, post w/ what kind of moped you have a bit more specifically and someone else might be able to help you much more.