Only one way to diagnose it. A compression test. Pull all the plugs. run a compression test. Record the numbers. Should all be within 10% of each other. If you find a low or dead one, put about a teaspoon of motor oil in that cylinder and try again. If it comes up, the rings are bad and the oil is sealing them. If it does not come up, you either have a hole in a piston or a burned valve. Simple to verify. Using your compression tester hose, apply compressed air to the dead or low cylinder with the valves to that cylinder closed. Air rushing out of the tailpipe indicates a bad exhaust valve. Air rushing out of the carburetor indicates a bad intake valve. Air rushing out of the valve cover oil filler indicates bad rings or a hole in a piston. Pretty simple, really. It's not rocket science.