I have a 1981 cm 400 which has sat for about 5 years. Believe it or not, when I added gas, the engine started up almost immediately and runs fine. After riding it for a short while, the clutch be gan to slip, and gradually became so bad, the bike will not go at all. Is there any internal adjustments I can make on the clutch, or must it be changed out completely in order to operate properly.