James,
Wish I could help more but I've never heard this question discussed before. Worn cylinders usually cause such problems as blowby, pistons pounding or rattling, shattered piston skirts, misfiring and dramatic loss of horsepower. I'm just guessing here but I dont think lean mixtures would be a result of worn cylinders. If crankcase pressure was leaking by the rings into the cylinder, it would be the proper fuel/air ratio. If burned gases were leaking into crankcase from the cylinder, It would be recycled back into the cylinder and possibly cause misfiring. Also, if the fresh fuel/air mix was being drawn up past the rings, you would get all sorts of detonation and preignition because the spark plugs fire not only at the top of the stroke but at the bottom of the stroke as well in a Rotax.
I know this in no way answers your question but its all I've got. My only recommendation would be that since you know the cylinders are worn beyond limits, and your friend has new cylinders. Put them in and see what happens.
Bill.
