What would cause oil blow-by to the ventilation tube on a 1973 Harley FXE?

If oil continuously blows out the crankcase vent tube (more than a drop now & then) it could mean either the oil tank is overfilled or the breather gear's timing mark is misaligned with the cam gear's mark.

If the crankcase only spews oil when the bike is started after sitting unused for a few days, then it means oil is seeping from the tank into the crankcase due to the check valve in the oil pump either having a speck of dirt holding it partially open or having a damaged ball and/or seat.