A update, So a just encase mice had carted nuts up the silencer and blocking that, I dropped it off and found it was quite clean inside, but end was rotted so fixed that, and ran it up for few minutes and what is failing got much worse and even turning it over by hand makes a good thot sound. so some part of cam lobe has gone or the follower, exhaust and injection timing is still normal. so needs a replacement cam and followers. Think its made of cast iron so not good to weld and steel would be easier but bit of trial and error get that exact and probably get distortion and even a small bit on the injection timing makes a lot.
