You can adjust the clutch at the sprocket cover you know refer to the haynes manual
Exactly, assumed you meant this adjuster was at max also. It's behind a rubber gromit thingy on the front sprocket cover as mentioned.
Check Haynes but basically you wind the handlebar adjuster all the way back in, slacken the nut at the sprocket adjuster and then turn the screw (check Haynes for which way) until you feel a little pressure. Then turn back a quarter turn and tighten the nut.
If all that fails then change the cable