CM 871 getting 3.6 MPG

[Rawze]

Another thing that comes to mind is compression loss in the engine. Might not be a bad idea to loosen up all the rockers so that all the valves are closed and then pressurize the intake manifold to about 30 psi. See if the intake holds pressure and does not drop off. Listen for air flowing into a cylinder with all the valves closed. - Then do the same on the exhaust side. There should not be any pressure drop for several minutes (on either side) on a good head.