The clutch pushrod endplay adjustment under the small access cover is VERY important. I would replace all the parts in there (the adjusting screw/ramp plate/ball bearing retaining plate/clutch release. From what you're describing the ramps are worn and the release is going over center and sticking in the fully disengaged position. With the new parts installed, first give the clutch cable full slack (screw the middle cable adjuster to its minimum length). Turn the adjusting screw clockwise until it turns freely if it isn't already at that point. Turn it counterclockwise until you feel the release engaging the pushrod (the screw will get hard to turn). Back it off 1/4 turn and lock the nut. Take up the majority of the cable slack with the middle adjuster on the cable, and then fine tune it with the adjuster on the clutch lever. Always have some free play in the clutch lever travel or you will smoke your clutch/release bearing and pushrod seal.