The CM91 died and will not start at all. On close inspection it sounded like there was a compression leak through the intake valve. I could hear hissing through the carburetor on both intake and compression strokes. Not good! The top end came apart once again. Last time I left the cylinder in place, but this time the top end will get a full rebuild. The cylinder is heavily pitted from previous corrosion and will need to be bored. I will need to have a machine shop tell me how far oversize they will need to go to remove the pits before I can order a new piston and rings.
Moving on to the head, I flipped it upside down and filled the chamber with alcohol which immediately ran straight out the intake port. Shining a flashlight at the chamber from the piston side, I could see light when I looked into the intake port. As expected, the intake valve appears to be the culprit. After removing the valve springs and disassembling the valves, I could see the intake valve seat is very rough. The exhaust valve seat is not much better, and I’m surprised that it wasn’t leaking as well. Hopefully, the seats can be recut and won’t need to be replaced. I also discovered that the inner valve spring on the exhaust valve was broken into several pieces, and the valve stem seal/seat was destroyed. Remarkably, all of the pieces seem to have been contained by the outer spring, preventing any other damage. Time to source some new parts and find a machine shop.
