To see if the idle is too high, put the bike on the center stand and note if the rear wheel spins at idle. At some low engine RPM the centrifugal clutch will make the bike move forward. Idle speed must be below this RPM.
Idle speed is usually controlled by a little screw on the carburetor. This particular screw pokes through and touches the throttle linkage. The tip of the screw holds the throttle open slightly.
If you turn the screw one way it opens the throttle more and the idle speed gets faster. Turn it the other way and the engine slows down.
There is probably one more screw right on the carburetor that adjusts the idle's fuel-air mixture. If both screws are adjusted properly, the engine will have a nice, slow smooth idle. Refer to the bikes owner's manual for directions on how to adjust idle.
One thing to keep in mind is if you adjust the idle speed when the engine is cold, it will probably die when it's warm and you come to a stop.. So get the bike good and hot before adjuting idle.