Well, I have confirmed that the problem is with the carb. Without touching anything else I put the concentric that I have been running for years, Started first kick and runs cleanly!! The monobloc seems fine, as I said I rebuilt it using correct jets slide etc etc. The floatbowl wouldn't seal so I put on an extended Bowl. (could this be a factor??) I will set it up on the bench and see if I can trace the problem. Incidentally I had gone through all the ignition, points , slip ring, Plugs, caps and Mag pickups when the problem started. definitely a puzzle.