Does the throttle appear to function properly while riding?
If so unscrew the idle adjust screw (left side of carb when sitting on bike) a half turn at a time until your engine idles properly.
If not check your cable, any kinks or sharp bends can cause the cable to bind, leaving you stuck at full throttle. You can also remove the needle and slide and see if it moves correctly with the throttle, and check for anything that may be blocking it.
Also if this isn't a new build, your throttle cable may be frayed.
If all else fails or you can't get a stable idle, check for air leaks on the intake. Check the intake mount, and spray a bit of starting fluid or carb cleaner around intake (avoid letting any into the air cleaner) while the engine's running and see if the speed changes.