[Arizoni]

IMHO in a properly vented system the downward movement of the piston quickly expels most of the air in the crankcase. 
This partial vacuum greatly lowers the boiling point of any water (a product of combustion) that is in the crankcase or mixed into the oil.  Given enough vacuum, water will boil at room temperature and although this much vacuum is not created in the crankcase the boiling point of the water can easily be reached by the normal oil temperatures in the engine.
The resulting "steam" is blown out thru the vent along with any gasses that have gotten past the pistons rings.

Without water in the oil system the mayonnaise like mixture of water and oil will not exist.

This is good because the oil pump cannot suck the mayo like stuff  thru any filter gauze or even the oil pickup tube.

[t120rbullet]

But that person would also have to explain to me the reason that having the puking mayo on the ground is a good thing. 

Better on the ground than in the engine, I guess ? Maybe ?
All the older engines I have and had never had a mayo issue.  I have a 04 engine in the white bitch now with the breather off the top of the oil tank and the screw-in oil cap and there always is a layer of mayo on top of the oil. Gives some color to the dipstick when checking the oil.
CJ

[Rockdodger]

This has been a really interesting and informative discussion. I'm far enough along with my "experiments" to be convinced that the problem with my '04 Bullet Sixty-5 was too much pressure in the oil tank. Running the dipstick in the first stop position stopped it from puking oil out of the catchcan vent. Running a traditional duckbill from the crankcase and venting the timing case return thru the catchcan to the atmosphere appears to have eliminated the problem of leaking oil at the headgasket.. I now believe that my problems started when I installed the K&N pancake filter. The old airbox had a nipple that was attached to the catchcan vent. This created a slight negative pressure that kept air flowing through the oil tank. With no good place to attach the vent hose to the K&N, I left it off. The original duckbill which ventilated the crankcase into the catchcan apparently was then pressurizing the oil tank, timing case, tappet cover and valve covers, causing various small oil leaks, especially on the pushrod side of the head gasket. At some point the vent in my dipstick, must have gotten partially blocked making the problem of over-pressurization worse.   

[Blltrdr]

This has been a really interesting and informative discussion. I'm far enough along with my "experiments" to be convinced that the problem with my '04 Bullet Sixty-5 was too much pressure in the oil tank. Running the dipstick in the first stop position stopped it from puking oil out of the catchcan vent. Running a traditional duckbill from the crankcase and venting the timing case return thru the catchcan to the atmosphere appears to have eliminated the problem of leaking oil at the headgasket.. I now believe that my problems started when I installed the K&N pancake filter. The old airbox had a nipple that was attached to the catchcan vent. This created a slight negative pressure that kept air flowing through the oil tank. With no good place to attach the vent hose to the K&N, I left it off. The original duckbill which ventilated the crankcase into the catchcan apparently was then pressurizing the oil tank, timing case, tappet cover and valve covers, causing various small oil leaks, especially on the pushrod side of the head gasket. At some point the vent in my dipstick, must have gotten partially blocked making the problem of over-pressurization worse.   

The problem with the revised breather system is that it is trying to vent the pressure built up in the crankcase through a little hole between the crankcase and the oil reservoir up through the top of the oil reservoir through a hose that is bent in the wrong direction up to the catch can and out the duckbill tip located in the catch can. This screwed up design has caused many problems and can only be alleviated somewhat. The proper fix is to do the mod to return the breather to the original position it had been in for 50 yrs. RD I would suggest you do a forum search and read the miles of posts on this subject, especially the many posts written by ace.cafe.

[Rockdodger]

Blltrdr, you are at least 100% correct. My current configuration is the old-style duckbill in the traditional place. On my bike the factory did a pretty good job of applying an automotive-style positive crankcase ventilation (PCV) system which adequately ventilated both the crankcase and oil tank. I had no problems until I added the K&N filter without making provision for excess pressure to be relieved by a vent hose to the air cleaner, as with the stock '04 Bullet Sixty-5. I think the folks who have had problems with cream gravy clogging up their catchcan have a different vent system (i.e. it sounds like some bikes have a duckbill or checkvalve on the oil tank fitting instead of the crankcase fitting.)

[Blltrdr]

Well the thing I notice the most since i changed over to the early model oil cap is......... I have no more mayo like goo spewing on the ground from my duckbill breather routed out the back of the rear fender. This is very interesting to me! Before the change from the screw on non-vented cap to the old style vented cap I had a blonde colored mayo blurped out onto the ground everywhere I stopped the bike for a period. Now I might have a little oil mist on the ground but no mayo. I think there is something to Ice's comments. Is it just his theory or is it RE engineering speak? I think it may be the latter. The bike runs great, maybe better than ever. I will keep running with the cap in the #1 position until I someone proves to me beyond a doubt that it is wrong. But that person would also have to explain to me the reason that having the puking mayo on the ground is a good thing. 

Another update since changing over to the old style dipstick cap with my reverted breather system.

Since reading Ice's post on the dipstick cap setting and completing the mod I have to say that the performance has changed in very good ways. First off like I mentioned before, no more mayo out the back from the duckbill. Every time I would ride my bike and park it when I came back there would be a big mayo goober on the ground. Now it seems to be nothing dripping from the duckbill. I also notice a difference when I throttle on at high speed. Before it seemed to pick up OK but would seem like it had to work hard to get over 60 and now seems to pull hard past 60 and seems willing to go much farther. Not like I'm going to ride that fast but before it seemed like I had a governor on the bike and now it doesn't. 

[Arizoni]

This is good to know.  Thanks for the update.