Hi there
First off, If the gear shift or nearside pipe is smoking more than the offside it certainly wont be gearbox oil contaminating it,and the rules for 21 and 28 motors on gear oil sucking in through the offside crankseal are the same for both engines, it would smoke from the offside pipe if this was your issue and when it does you can smell the fumes are much stronger and thicker than the normal 2 stroke fumes,and the smoke certainly wouldnt improve after a spanking for 20 mins it gets worse,
When cold does the bike start easily and run on both cylinders straight away ? if so I would be surprised if your crankseals are faulty,
Good luck in your quest,
jonmac
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John