I am not no mechanic, so I might be wrong, but if there is a return spring that might be stretched so it doesnt have the tension it needs to return itself back to idle. ( this happened on my minibike before) check that, and also take the cable off and check for kinks or binding, and lastly spend the few bucks and get a cable lube thing. It might not be the problem, but they do come in handy.
86 Honda 200sx (needs alot of work)
82 XR200R (runs great, now the daily rider)
91 acura legend junker, selling to get a truck
2001 GSXR600 Part out, PM for info or for parts.