I would also repeat the test with the ufm off and the rear exhaust spring cap removed,just to make sure that the cap does not hold the valve open a smidge.
If you want to save a bit of drama (possibly) you can cut a rough open-ended spanner out of 1/8" steel plate and just ease off the inspection cap about half a turn which will give you about .025" clearance to check for this fault. I think this should work on a Twin, I've done it on a Comet inlet valve spring cap.
Update 2. Oil tank off, measured under the cap with plasticene does not seem to be a problem between 1 and 2mm gap. Also did another pressure test with the cap off, same problem leaking air badly.Tapped the valve with rubber hammer to lift of the seat but still same no improvement. Next step head off and valve out.
Head off and valve out, not looking good, valve is loose in the bottom guide and also is not concentric / perpendicular to the valve seat which can be seen worn off centre. Guessing at least will need new oversize guide and seat if it can be done. Not surprised it is leaking.
My Wife's Son had a Rubbish French car, And the cam belt broke, I said I would put a new belt on, But no more, He said it was down on power, I said good !.
He had a bent valve, Every morning I could hear it getting better, When he came to sell it some time later, The buyer said how good the engine sounded !!. Cheers Bill.
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