[rotorwrench]

Under normal crankcase ventilation operation the air is supposed to go in on the oil filler cap and come out at the tip of the road draft tube but excessive blow by at the rings can overwhelm the whole system. As was mentioned, a blockage of the road draft part of the system will make a new vent out of the filler cap. It pays to have an original oil cap too. Some of the other caps that will fit on there don't work out as well.