Have you got a good tappet clearance with the valves closed? Spark plug loose? Take the plug out and shine a small torch in and just have a look first. Remove oil dipstick, put your thumb over the hole, pull it over, can you feel pressure in the crankcase? Can you hear where the compression is going?
Had a yamaha motorbike that smashed the gudgeon pin clip to pieces which gave me a floating pin that machined its way into the barrel, hope its not that serious, just thought i would say.
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