[SpudRider]

I suggest you keep the plug in the bottom tube of the oil separator assembly. The plug helps to keep dust from being sucked into the foam of the oil separator, which will shorten its life. The plug also helps to prevent dust from being sucked into the crankcase itself. Also, if the running engine is submerged on its left side, the plug will help to keep water from being sucked into the crankcase. This is why most crankcase vent tubes terminate in the air box. The air box is a dust free environment. Also, if the bike is submerged, the vent is located higher, and is hopefully located above the water line. Finally, if the plug is water tight, the plugged tube allows you to collect the contaminated liquid for proper disposal, along with your used engine oil, instead of spreading the contaminated liquid into the environment.