1989 XJ600

I have a problem with this bike with idle speed, when bike is first warm
when you close the throttle the engine revs will drop back to 1200 but once
fully warm after riding for 10 to 15mins when the throttle is closed the
revs will stay at 3000 and sometime just slowly drop back to 1200. Has
anyone any suggestions, I have had the carbs out and back but have not found
any damage etc to the manifolds. All help appreciated while I think on I
have also tried syncronising the carbs but this has made no difference.

Thanks in advance JEB