It could be the throttle cable at fault. If the cable can't move freely through the lining then it's time to buy a new cable. To be sure though, first disconnect the throttle cable from the throttle handle. If the handle twists freely then you know that it is definitely the cable. If the handle doesn't twist then order a new handle and replace it. Cables can fray so eventually they need replacing. Also if they are not lubricated from time to time they can also stiffen. Check where the throttle cable enters the carburettor too to see if there is any restriction there. You will have to remove the seat and side panels to get to the engine to replace the cable. Pull the cable above the carb to see if is can be pulled easily or not. If there cable doesn't pull up and spring back easily then is could be a spring problem in the carb. The good news is that any of those problems are easy and cheap to fix and can be done by yourself.
Angela., February 2008