I don't think you see much airflow (especially compared to the intake manifold) from the nipples in the back plate to the intake manifold. The only air admitted to the crankcase is bloiwby from the piston/cyl.
While I am sure that there is a steady air movement from the cylinder there isn't the torrent of air moving thru the crank case that you see with a 2-stroke engine. I think you will have a better understanding of just what these nipples to if you think of the center backplate to carb connection as a pressure fluctuation line to operate the diaphragm type fuel pump in the carb (hence no air flow.) The nipple on the front bottom ot the crank case as a low pressure well to encourage some of blow by oil in the crank case to flow towards the front ball bearing (hence no air flow.) Finally think of the bottom nipple as a sump to collect excess blowby oil (if any) and a pcv (positive crankcase vent) connection to the middle of the intake manifold, If your engine has good compression there isn't really much of a supply of air to the crank case in the first place and compared to the diameter of the intake, not much air COULD pass through that oil clogged tube into the manifold. D.H.