| Shazaams post is well founded. There's nothing wrong with the stock system at all. If you look deeper into the "physics" of crankcase venting then the K&N is the wrong way to go.
More impoprtantly the issue of oil misting over the back wheel is the biggest issue. The engines can and do breath quite heavily, easily pumping some, if only a small amount of oil to the airbox. I've seen quite a few and know of a couple that have resulted in off's.
I wouldn't sell them. Won't fit them and advise against them. | 
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