Try cleaning and lubing the throttle cable to see if it helps your return issue. Or disconnect them from the bell crank at the back and see how it responds manually. If it's in your cables, an old spark plug boot fits nicely over the cables at the top and makes a reservoir for your favourite cleaner. Place a rag under the cables at the carbs and work them back and forth until they run freely with the lubricant/ cleaner all the way through. Avoid anything with brake cleaner/acetone for the sake of your paint. It could be in the throttle sleeve at the grips too.