Starting up old engines.

[roverfj1200]

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.

[Dieselman7.3]

Worn rings or valves? Could be closing the tolerance one warmed up and stops smoking.  Just be my guess

[Motofun]

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.

[red]

Rover,

Yamaha makes a dedicated additive called Ring-Free, which may or may not help.  Directions are on the container.
.

[Pat Conlon]

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.