The first thing that comes to mind is the crankcase ventilation valve. Here is what the installation should look like (compliments of Mark Weller, who took the photo of Tim Toth's beautiful motor). If the valve that runs from the crankcase to the carb is not working properly (is stuck) then pressure can build up in the crankcase and the vent becomes the oil breather. It's supposed to breathe, but not in an outward direction.
Do you have aftermarket carbs, or aftermarket valve covers (some of the latter don't have the internal baffles like the stock covers do). Is there anything not stock on the motor? First thing I'd do is to look at the crankcase vent system. It's pretty simple, internal parts just slide back and forth.
regards, Paul
