Oh -- I didn't know that you were not talking of the Bogen badged Lencos -- since Bogen used Bogen Presto as their name for a while, or seemed to, I thought you were just refering to a Lenco. But I think the clue to your unusual results are in your unusual set up. I suspect that the narrow rim of the Lenco idler really is too small a contact area for your set up, and as it wears, its behavior changes, not because of diameter, but because of something to do with friction and surface area. In fact, the wearing would suggest exactly that -- I think most idlers suffer from glazing and flat spots, but not from actual wear, so something unusual is going on. I don't mean to be contentious -- I'm just guessing on the basis of a very small amount of information. But the general argument that idler diameter doesn't matter seems unassailable to me -- for what ever reason, your situation is a different issue.