[Dave at Pelican Parts]

This hose helps get rid of crankcase oil vapors, and keeps the case from building up air pressure inside of it.

I would replace the cork gasket under the oil filler/breather box, and the O-ring and seal under the oil cap. These need to be (reasonably) air-tight or you will get an air leak into the intake through them. I believe there may also be an O-ring between the oil filler funnel and the filler/breather box. Check that for leaks as well.

--DD