I just finished disassembling the top end on my 1971 xs1-b and found a burned exhaust valve. What will cause this? The bike has been burning oil,and I suspected worn valve guides. This doesn't seem to be the case, however the valve guide seals have totally disintegrated. The piston rings seem to be worn also. The cylinders and pistons are in great shape. Timing chain is totally worn out (stretched).