One of the joys of early retirement is finding the time to catch up on some reading. Thus in "Diamond Flight", the story of British Midland by Bill Gunston, there is an explanation of the problems found on the Merlin 724 used in the Derby Airways DC-4Ms. Apparently incorrect settings of rpm and manifold pressure for a given pressure altitude gave rise to burnt exhaust valves. This was exacerbated by the use of a detergent oil which put a sticky residue on the valve stems which prevented rotation so the exhaust flame always attacked the same area round the tulip head, literally burning away that portion.
As a result Derby Airways suffered a long series of inflight featherings and exploded exhaust systems. I can't imagine that the York wouldn't have had similar problems.