Peel back the handle bar grip if you can. If it's hardened up over the years you may have to cut it off.
Underneath the revealed metal twistgrip nearest to the brake lever mount you'll see a small cross head screw running in a cut out slot.
Remove this screw and the twist grip which runs on an internal helix will be able to be removed from the handlebar.
If you need to remove the throttle cable, you may find it easier to remove the horn switch button. To do this remove the handbrake lever and you'll see a small hole underneath which gives access to a small cross head screw.
If you do take the throttle cable out, tape a piece of cord / wire / or another cable to the carb end before you pull the cable out of the handlebars. This will make replacing the throttle cable a lot easier.
