Probably have clogged low speed jets. Try a full can of Seafoam at your next fill-up.
Do the simple stuff first
+1....agreed
The chemical solution is no substitute for a thorough cleaning in my experience. Been down this road myself and have helped 3 other buddies remove/clean their carbs and all of us have been thrilled with the results.
If you plan to keep this bike for a while, then I recommend just doing it right by cleaning the carbs and installing new slow jets. It's also a good opportunity to replace or remove vacuum lines, install new fuel lines, and replace pilot & intake orings. If you do it this way you should be good for another 10 years with proper fuel storage habits during long periods of layup.
Good luck....feel free to pm if you have any questions.