A stuck valve will usually be NOISY also. If it is a stuck valve. You can remove the valve cover. The stuck valve tip will be lower than the others. Old fashioned cure. Tap valve with a hammer. Run motor and pour kerosene/fuel oil slowly into carb. SMOKES like hell! but will loosen up stuck valve with a bit of work.
I would take a compression reading and find out if it is a valve or a blown head gasket.