its clutch drag, when you say adjusted is that just at the handle bars or the full adjustment?
you could have a sticky cable, so lube it!
there may be some rust on the clutch rod preventing full movement.
or you might have rusted plates and just need to ride the bike
or the plates have warped preventing them opening
oe there may be grooves on the clutch basket preventing the plates moving
there's a lot to go on to keep you busy for a while