Yes, I'd first look at the fast idle screw.
Next, is your timing way advanced? You say it runs fine otherwise so perhaps your timing is ok.
Here's what I'd do...take the carb off. Inspect the primary throttle plates (butterflies). You'll probably see they aren't completely shut as they should be with the idle screw not touching the throttle lever. See if you can determine what's preventing them from closing. Perhaps, the geometry between the two primary butterflies on the throttle shaft and the carb base has changed. If you suspect this, remove the carb base (the lower part of the carb), and carefully loosen the four screws the that attach the throttle butterflies to the throttle shaft and allow the plates to re-seat themselves (DO NOT REMOVE THESE SCREWS...only loosen, with WD-40) about 3/4 of a turn. Once they've 'centered' themselves, retighten the screws.
Upon the above inspection you determine the butterflies are completely closed, I'd look for a stuck power piston...stuck open, which can elevate your idle...also makes things rich.
See if the above works...if not, e-mail me...I have a few more ideas if these don't work.