There was universal concern from the other owners that one of the valves was leaking. As the OP had hinted, a compression test would be a good way of testing that but compression testers usually screw in to the glow plug housing and the Yanmar GM series doesn’t have those. GM’s are notorious for valves leaking the cylinder head – it is well known as a weak link in that engine series and that’s were most owners focussed their advice on.
One owner, however, suspected that the injection timing may not be correct and suggested some additional diagnostic tests;
Did you put the same number and thickness of brass shims under the injection pump when you replaced it? That’s what controls injection timing. Is there any exhaust smoke when it starts first time on a cold start? You can run the engine and a slightly high idle Enough to keep it running, and activate the decompression release is one of the time to see how that affects the engine. It should run much worse when you activate decompression levers one at a time and if it doesn’t run much worse that’s the offending cylinder.D. T.
Another asked about the exhaust elbow but only as the root cause of the valve problems.
Have you looked inside the exhaust elbow? The previous owner of my GM had replaced the elbow but evidently before that water had been backing up into the head. One valve was badly corroded as well as the seat. My engine would only start on one cylinder. I pulled head. Replaced valve and seat. Had the head overhauled and reinstalled. You can pull and reinstall yourself just bring head to a machine shop that rebuilds heads. Runs great now.J.W.
This contributor provided some pictures of what a bad one looks like…
So the consensus was that a top-end rebuild was necessary. The final piece of useful advice was to be very careful when removing and reinstalling the head…
You need to remove and reinstall the head bolts/nuts in the proper order according to the service manual. Need a large torque wrench when reinstalling.J.W.