Could be sticky gummed up linkages or your return spring just needs replacing. Spray on some carb cleaner to free up any linkages. Return springs are easy to find and fix, but don't put a really heavy spring on there, too much tension will wear the heck out of the throttle shaft bushings, (if they are'nt worn already). Your carb is just probably dirty, you did'nt say if the carb was rebuilt at any time but I'm assuming it's probably not been. Anyway, try the simple things first, I always say. Hope this helps.
