Crank case breather

[rev_mook]

Hi Gang!  I started to notice some oil at the junction where the starter cover fits to the crank case.  Under the starter cover, near the migrated oil, there is the crank case breather, which is where I suspect the oil is coming from.

My question is, why would oil be coming from this breather?  How much air is this designed to pass?  

Another thing I noticed when changing the oil, is there seems to be a ton of air coming from the dip stick hole.  Shouldn't the breather take care of this pressure?  Perhaps I have some bad rings on one of the cylinders causing excessive pressure?  Please see videos attached.  

https://www.youtube.com/watch?v=wmgQgkbs5hA

https://www.youtube.com/watch?v=n53zMJwPYwE

Any ideas?

[mrclubike]

The crank case breathes as the pistons go in and out.
So the crankcase   gets pressurized  as the pistons come in releasing pressure (plus a small amount of oil vapor)  thru the breather valve but then the CC goes to a vacuum or lower pressure as the pistons go out  
They go in and out together not opposite of each other like a 4 cylinder.
If you remove your filler cap you defeat the breather valve because the The air is going to go in and out of the filler hole easier than thru the breather.
On a lot of single and twin cylinder  engine  you will get a face full of oil when you do that  
All normal.
The oil should be making its way out of the breather tube into the carb inlet.
So check the flatness of the breather cover and reseal it.
If possible throw the gasket away and use a heavy duty black or grey silicone.  
Make sure your breather hoses and clamps are all good also.
I don't know for sure but i wonder if a leaking rear main seal could sling oil up there
Some of the others here with more experience with this engine may know of other oil leaks that show up from there
Most airhead owners run the oil level half way between the FULL mark and the ADD mark

[Barry]

My question is, why would oil be coming from this breather?How much air is this designed to pass?

Another thing I noticed when changing the oil, is there seems to be a ton of air coming from the dip stick hole.Shouldn't the breather take care of this pressure?Perhaps I have some bad rings on one of the cylinders causing excessive pressure?  

A small amount of oil from the crankcase breather is normal.

The function of the breather is to pump down the crankcase to a vacuum which will minimise the power and oil loss from air being constantly pumped in and out.  As the pistons move in air is expelled by the breather and when the pistons are on the way out again the breather is closed which will produce a vacuum. On the next inward stroke there is then less air to expel.

So in an idealised scenario you can see that on average the crankcase would operate under a vacuum.  Problem is there are other potential sources of crankcase pressurisation. Some blowby of combustion gases past the rings is inevitable.  Also any place there is a seal like the crankshaft rear main seal it's possible that air can enter. There is an infamous Turkey gobble noise that is reckoned to be a symptom of leaky seals. If you remove the dipstick that's a major air leak and the breather cannot possibly cope so it will just suck air in and out and blow oil all over the place.

When the breather is having to fighting these other issues some oil loss in inevitable but you can minimise it by running the oil level no more than half way up the dipstick. This increases the air volume in the crankcase which reduces pressure fluctuations giving the breather more chance of coping.

[montmil]

My initial thought is the crankcase is over filled.

All three of my Airheads are maintained with the dipstick level showing halfway twix full and low marks. This is a help in keeping oil out of the carbs. I dislike the idea of venting crankcase pressure and oil blowby into the fuel system. Have also seen a few Airheads running custom -and some not-so-custom- catch cans for oil collection.

[Bob_Roller]

If you remove the metal plate of the breather and remove it, the hole that lets the pressure out, is quite small .
Is the thin piece of metal that acts as the valve, still there, it's much like a thin feeler gauge .
As has been mentioned, the paper gasket may be hardened with age and heat exposure, get or make a replacement gasket and use oil resistant sealant .
The breather hose may well be hardened as well and the worm gear clamp can't make a good seal anymore where the hose goes onto the breather housing .