The crankcase is connected to the intake tract which
..i have an open filter on my a50..which disconnects the valve cover's vent tube from the airbox...
..with the tube connected to the airbox, it creates a suction, which actually draws oil vapor into the airbox from the valve cover...
..most of the vapor condensates there, and can be drained off, but a certain small amount enters the air flow into the carb...