I have been hearing about the stuck breather problem but had never seen one till yesterday. It was interesting.
I had the head off a junked mower to inspect the cylinder bore. Turning the engine by the flywheel created enough positive pressure in the crankcase forcing large amounts of oil out the lower valve. Think pump.
There is no way this engine could run as it would instantly foul the plug. Blowing into the breather cleared the problem.
This is interesting enough that I may make a video.