Hi splittie65, Sorry for the delay. Had to dig into the archives...
Disc lets air out as pistons fall, blocks air coming back in as pistons rise. Intake cam is hollow. Has 1/4" hole drilled just to right of the right cam lobe to allow free passage of air to disc. The rotating cam may also act as a centrifuge to help separate oil from blow by. A very clever arrangement.
I did a diagram & testing method for disc type breather. Since breather plate & cam disc have to holes (more like triangles) the cam lines up breathing every 1/2 turn cam.
The holes in motor case are vertical. The drive slots in cam are offset at about 1:30 o'clock (between 1&2). The holes in cam disc are offset from drive tangs. So... when holes in cam disc align with holes in motor case exactly, cam lobes point to 10:30 & 4:30 looking from left side of motor. No matter the way cam disc installed as long as tangs enter cam.
This is real life test. Hook long 3/8" hose to breather pipe under motor. Remove adjuster access cap for left intake rocker. Blow into hose with mouth. Rotate engine normal direction with wrench
or rear wheel.
Breather disc will pass air just before left intake valve fully open.
Then disc is fully lined up to holes just as intake valve is closing & you feel air passing freely.
Air passage stops just before intake valve is closed.
However.... It passes air again when valve is fully closed.... Don't let this confuse you. Remember it's fully open every half turn of cam. Keep blowing as you turn & that tests the 2nd have turn. I didn't record rocker positions for that.
The passage & blockage of air is very obvious when you do this test. When it's off, it's obviously off & it stops the air. If it's not obviously off, that is a fault. If it feels restricted, that's a fault.
The idea is this. Air exits crank case when pistons fall. Blocks air coming back in as pistons rise. So the crank case runs in negative pressure as pistons rise. (A vacuum).This is why crank sprocket seal has spring towards sprocket. To prevent primary oil from being sucked into crankcase while pistons rise.
This test is easy to do with motor in frame or on bench. Ok... not that easy to hook hose to breather motor in frame, but really no big deal. I've not done it, but I think hook hose to breather pipe at rear fender if all hoses are sound.
On later primary breather bikes, you can feel the negative pressure as pistons rise when you put finger over end of fender breather pipe. Fun stuff!