I had a similar problem on an XS250 except it wouldn't come down from 3000rpm. Cleaned the carbs - no difference. Tried another pair of carbs (ebay) - no difference.
I noticed that the inlet stubs looked perished from the outside but the cracks didn't seem to go all the way through but I bought another pair of stubs (ebay), made some gaskets from gasket paper and fitted them with high temperature silicone RTV (Loctite 5920). When the silicone was set I refitted the original carbs and the problem was solved.
HTH
