One easy thing to check is to see if the bike has a throttle lock (set screw) on it. Look at the bottom of handlebar throttle housing if you see a silver screw going in at an angle you have one. If you see nothing but the black housing you don't have one.
If you have one, sit on the bike and reach under the handlebar and turn the screw
clockwise this will loosen it (because the screw is upside down). Loosen it until the throttle returns freely.
Different scenario:
I used to have a service video and the guy in the video said to remove the screw on the top of handlebar throttle housing and remove the cover. You will see a little ring gear in some grease. take it out, clean it and the gear on the twistgrip and regrease. NOTE THE ALIGNMENT MARKS! You'll see etched into the gear and twistgrip gear lines that you have to line up. Make sure they are lined up or the throttle won't open all the way.
I never lubled the cable itself and mine's been fine.
