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I was just reading the first post.  It is unusual that the motor runs well for a bit then goes onto one cylinder. It sounds to me as though it might be spark rather than jetting.  One of the plugs might of a different heat range to the other and might be fouling. If the motor coughs and spits when the choke is taken off, after the motor is warm - that is a sign of lean-ness.  I would look at the carbon on the porcelain in the plugs. If one hasn't burned the carbon off properly it might have a different heat range. I would also take note if one of the plugs looks greasy - might be a rings problem. If you place your hand behind the silencers when the motor is running on one cylinder, you should be able to tell which cylinder has gone duff.