The breather valves are one way (exit only) so oil would not flow back into the heads very efficiently. You'll just end up with oil mist all over the top of the engine/frame.

 

And you are correct. Tin men BS and even the mother ship have said this however for of course different reasons.
If you overfilled it enough to get oil all the way up into the head where the vent is, the downward travel of the pistons would blow the seals out.
I have never see a Harley or any other two cylinder motors that do not drip a little oil into the air filter box.
It's actually too heavy to be drawn into the air intake throttle body.
It hurts nothing.

 

The oil pumped into the heads is not really the oil going out the breathers.
The air going out the breathers is air being pushed out of the crankcase due to the pistons coming down.
The violent act of the pistons moving and the crank spinning (and blowby) causes oil to be picked up and vaporized (mist) within the case.
As the pistons come down that air is compressed and it seeks out an area of low pressure to travel to.
The air/oil vapor travels through the right side crank bearing into the cam case and up the pushrod tubes. Once in the head it passes out through the breathers.

As stated, as the pistons come down, the air in the case will compress.
If the case itself is large, there is more air space available to absorb the compression (like blowing air into a 50L trash bag), thus less air compression and less air/oil vapor coming out of the breathers.
With a small engine case you will get more air compression (think blowing air into a balloon) and thus more air pressure and more air out the breathers.
Our engines have a small engine case but it's air space capacity is increased by it's connection to the oil pan. Available air space for compression equals engine case + oil pan.

So... when you over fill the oil pan you decrease the air space available for air compression and thus increase case pressure and the air blowing out the breathers.

 

So if it's like blowing into a 50L trash bag, what is 1/2 a L more space going to do for turbulence / pressure?

 

[QUOTE=RIPSAW;17088170]And you are correct. Tin men BS and even the mother ship have said this however for of course different reasons.
If you overfilled it enough to get oil all the way up into the head where the vent is, the downward travel of the pistons would blow the seals out.
I have never see a Harley or any other two cylinder motors that do not drip a little oil into the air filter box.
It's actually too heavy to be drawn into the air intake throttle body.
It hurts nothing.[/QUOTE]

Except for the fact that by the end of the riding season my exhaust was getting covered in dirty oil that would burn onto the chrome as I rode.
The picture below shows how much oil (I really wasn't ready for that much) would have been either burned in the combustion chamber with it's associated dead air (making zero power) or would have ended up in my air cleaner (as seemed to happen more often.) This is from a good running 2011 RGU when I emptied my catch can last year.

 

 

The popet valves after three 180 turn in the baffle and catch screen are one way out. So any tramp oil getting past them as a mist with the air is not going back. So as it falls out, and gets on the passage ways to the air filter box is what you see in the bottom.
My Yamaha has a cup area in under the vent opening in the airbox. Attached to the cup is a hose with a plug to actually drain it. Interesting that nowhere in the service manual mentioned it. At 10k, I was doing maintenance and realized it was not the carb overflow like you would see on a Harley. Also , the cup and hose at about 3 or so teaspoons in it at 10k. However, that was sense new and the 1100 V-Star turns about a 1000rpm higher the my Harley.
At 50k, I will see a few drips on the oil bag and exhaust if bike is ran hard after sitting thru the winter. Usually since I have an open filter, it just keeps the bottom wet and I wipe it's drool occasionally with a cloth.

 

The one-way action of the breathers is achieved through use of umbrella valves.

Try blowing into a breather tube and you'll see not much is going back into the head once it comes out.

Another function of having breathers in the first place is to expel moisture within the engine. You wouldn't want that milky water infused oil running back into the head.

 

Yup, it works. This is after 500 miles. Water in bottom of can and sponge contained milky oil.

 

Yea I've taken the breathers out of my air box, why wouldn't ya.