I recently fitted a second hand low mileage engine to my bike. This engine has sat un-started for many years. I first started the engine in the old bike and cycled it through a few warm up cool down cycles. Compression was down on 3 cylinders but was building when turned over with no plugs or carbs fitted. The engine blow some smoke at first but become less as I cycled it hot and cold. I then removed it and fitted it to the next bike and on start up blow a steady cloud of blue smoke for a few minutes. I allow it sometime to cool and started it again and there was no smoke to be seen. A test ride of 5 miles or so and the bike ran well with no sign of smoke. After 24 hours I started the engine and ran it for 2 to 3 minutes as it filled the shed with blue smoke. Shut down and allowed to stand for 5 minutes or so and on restart there was no smoke issue at all. This is not like valve seals where a puff of smoke is emitted on start up, the smoke continues for the whole warm up and only stops after a rest.
I feel this will fix itself with a few good rides but would be interested in what other peoples thoughts are as I have never seen this heavy smoking and then non before.
Worn rings or valves? Could be closing the tolerance one warmed up and stops smoking. Just be my guess
If it smokes after sitting all night but not when it's warmed up I would think that's an indication of the valve stem seals leaking.
Rover,
Yamaha makes a dedicated additive called Ring-Free, which may or may not help. Directions are on the container.
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I assume you did all the basics, checked the valve clearances, cleaned & balanced the carbs, etc, etc...
Don't fuss around, ride it....then ride it some more. No coddling. Ride the piss out of it.
Reevaluate after your 2nd oil change.