Topic: How does oil "smoke" get into the cam cover in the first place?

[mycb750k6]

I searched and read many threads on 'smoking' breather tubes on CB750s but I still don't get how the smoke gets in the cam cover to come out the breather in the first place? I'm baffled. I've seen a number of posts about this just recently but I was hoping someone could offer a simple explanation.

[heffay]

That area is open to the bottom end thru the cam chain area.
Also, the smoke is not from the combustion chamber, its typically just really hot oil steaming, get it hot enough and it will smoke.
If your valve guides are shot, could be combustion gases though.

[mycb750k6]

Thank you. That's a really good answer. I guess the question now is why would oil get hot enough to steam in one motor and not another, assuming the same oil.

[heffay]

My experience with steam/smoke from the breather is running the bike hard at high RPMs, then coming to a stop light or something, the bike will be really hot and not cooling at that point (we ride air cooled bikes, need airflow) so the breather starts to flow.

a clogged breather on one and open one on another would answer your question too.   

[mrrch]

Sometimes the type of baffling in the breather system can make a difference.

[CycleRanger]

Any engine with have some amount of "blow by" from the valve guides and pistons as well as some steam from condensation. Perfectly normal for any engine. 
Since ~1965 or so all cars in the US were required to use a system (PCV) to recirculate these blow by gasses back into the intake manifold to be "reburned" in the engine.  Motorcycles weren't required to have this until much later so the early SOHC's just vent to atmosphere.

[mycb750k6]

So in this case blow by from the pistons would travel up the valve guides into the breather?? That's why bad rings or worn pistons might cause this 'smoke'??? Yes/No?

[CycleRanger]

So in this case blow by from the pistons would travel up the valve guides into the breather?? That's why bad rings or worn pistons might cause this 'smoke' Yes/No?

The breather vents the entire case. Gasses escape past the piston rings into the case or out through the valve guides.
Yes, excessive amounts of smoke from the breather is a good indicator of worn rings or valve guides/seals.

Here's some additional info about CV systems in general.  http://en.wikipedia.org/wiki/Crankcase_ventilation_system

[mycb750k6]

OK, that finally makes sense to me. I'm a slow learner sometimes. Thx.

[CycleRanger]

Any blow-by past the rings will go into the case and be drawn up through the cam tunnel and then vented to atmosphere.