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Had the same problem on my Jeep. Was caused by a clogged PCV valve. On a motorcycle, it could be a clogged, pinched, or disconnected breather hose. Blow-by gases in the crankcase should be routed to the airbox for reburning. The crankcase is already pressurized by the pistons. The worse case scenario is excessive blow-by the rings which exceedes the normal pressure in the crankcase and sends an oil mist to the airbox.