[dmjlambert]

I was looking for the post and thought I had it bookmarked but I guess I can't find it now. Somebody here made an explanation that: normally the vent side with the flame arrestor is where fresh air goes into the engine, and the PCV valve side is where the blow by and other unburned fuel/oil saturated dirty air comes out of the engine. That dirty air is then sucked into the intake and burned as fuel. There are conditions of load especially as the engine gets more wear, there is more blow by and air will actually come out of both the fresh air inlet and the PCV valve. That is when it is useful to have the flame arrester and hose to the air cleaner, so that dirty air is also sucked into the carburetor and burned. If you use a breather, which is a minimal filter, that dirty air will come out and dirty the valve cover and engine compartment and stink up the cab.