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3 crucial aspects of accurate & repeatable balancing are location, location and location - i.e. how the part you are measuring the unbalance of is located on the thing which is spinning it.
When a clutch (or flywheel) is balanced on it's own, then it is balanced around the centre of rotation of the adaptor that it's mounted to. When it's fitted to the crankshaft then it's centre of rotation may be minutely different, with significant results.
Take the case of a clutch cover plate which is located on the adaptor (and later on the flywheel) by a number of dowels. The amount of unbalance measured and corrected for will depend on the accuracy of the position of the dowels on the adaptor, the accuracy of the position of the holes in the clutch cover plate, and the accuracy of the location of the holes on the dowels. Take the clutch off, turn it around and put it back on and you'll get a different amount & position of unbalance.
Similarly for the flywheel and the way it locates onto the adaptor it's balanced on and the crank it's subsequently attached to.
This is why you should balance the crank on it's own, add the front pulley & rebalance, add the flywheel & rebalance, and finally add the clutch and rebalance.
A part is only balanced around a particular physical centre of rotation - change that centre point (even by a thou) and it becomes (more) unbalanced. If you have a perfectly 5kg flywheel and it's centre of rotation moves by thousandth of an inch (apologies for mixing units!) then it'll become unbalanced to the tune of 127gmm.
Apologies for the ramble... as Denis says this sounds more like an oil supply problem.
Mike
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