I'm assuming you're using the right oil for the bike - using the wrong oil can cause clutch drag (or indeed, clutch slip).
Just my two scents, but mine doesn't do this. Back wheel is free - spins 2 times from a good push so the back brake definitely isn't binding.
If you're sure there's no more adjustment i would start with a new clutch cable, but order a Yamaha OEM one from Fowlers of Bristol, not an Ebay jobbie.