Was prepping boat for some cruising and had it idling at 1500 RPMs. Went back to do a quick visual and RPMs picked up. Raced back to throttle to see 3,800 or so on tack. Throttled back, heard knocking, immediately shut down. Inspected engine. No outward trouble signs ? no oil leaks, temp fine, no water in oil, spins freely. Started again, same clatter. Oh no. Compression check ? 180 on 3 cyls, 0 on one. Not even a gauge flutter while cranking with all plugs out/all ignition wires disconnected, throttle wide open. Took valve cover off. Cam looks good. Turns when cranking. Valve dampers move up and down on lobes. Don?t hear piston hitting anything. Non-interference I hope. But then what was clatter when running? Could a valve have dropped far enough to come in contact with piston? What would you do next? Leakdown? Pull head? Am I up poops creek? If bad piston, I do have a lead on a rebuilt 120 hp 4 banger. $1500 with outdrive.
