Landrover 4cyl had a very common problem of exhaust manifold burning through to inlet through hotbox, as i remember my first encounter nearly sent me MAAAD. Your manifold may have had a defect in the casting which decided to let go with torsional stresses through R and R. Spray aero start into inlet and see if revs pick up, your problem now seems to be related to unusual reasons, My second land rover was easy to faultfind, iunder stand your frustration Regards Jack mcIntyre.