Long and short of it, I ripped the head off as re-did my compression test and I got 320/380/410 on a warm engine. Enough of a variation to cause issues.
The valves where all burnt and crusty where it had been running like crap for a while. The previous owner of the engine had sorted the head a while back so it had done the valve damage pretty quickly. I had a chat with him, and we have no history of the engine. The thoughts are that it has a worn out bottom end, worn rings probably, as the bores looked good with no scoring/marks etc.
It was always grumpy when cold, even on 100% diesel and never seemed to make the power it should, which would explain all that. After figuring out that a bottom end rebuild would cost £1000+ I grabbed a 10 hour! D722 from an engine supplier in greece for £600 and it has now arrived.
Its got a few better features than the D850 it replaces. A better vortexing prechamber which should work with WVO well, and glow plugs that are self regulating, meaning I can afterglow them on very cold starts.
It has enough differences that bolting in to the bike will take a bit of modification, mainly to the mountings, the air intake piping, and the exhaust pipes. Hmm
A test run:
https://www.youtube.com/watch?v=JVOo0o18NOw
