I know most of the time guys use a filter on the valve cover breather line. I still have my 'PCV' (as I'll call it) cannister hooked up. The top outlet is open now which I need to deal with so it won't spit residue out on my pretty powder coated engine as it's pointing forward to it's left. May be able to rotate it 180 degrees but haven't looked into that yet. This normally is routed to the top of the air cleaner on later bikes for air pollution reburn I'd assume. Therefore it's not a pressurized system but rather just a relief mechanism that attempts to reburn unburned hydrocarbons that are now spitting out miniscule amounts from the top of my cannister.
There seems to be a lot of mystery about the recirculation system, how it works and what it is for. Folklore certainly doesn't do it justice and mostly applies negative connotations for its inclusion, which is simply as wrong a saying the earth is flat.
The device Jerry has called a PCV is really a water separator. PCV would be a "Positive Crankcase Ventilation" device, which it does not do.
Autos have this, and it applies engine vacuum to the crankcase, but there is also an inlet feed to the crankcase to provide fresh air to it as well, therefore positive ventilation.
The SOHC4 engine, does not have a fresh air supply to the crankcase. So, there is no positive ventilation. But, this is a good thing, explained later.
At the top of the CB750's air box is a fitting which provides two functions. Function one is to supply a partial vacuum to the recirculator to power it's operation. The second it to provide an exit path for accumulated vapors that the recirculator collects.
Why is there partial vacuum at the top of the 750 air box? Because it is located in the path between the engine intake valve and the air box inlet. The air box inlet is the first barrier to pressure equalization, so that is the first step toward the lower pressure at the intake valve. The next step is the filter membrane, which provides yet another step toward the lower pressure at the intake valve. The top fitting on the CB750 pressure wise, is between the filter and the carb inlet and the filter. The amount of pressure drop varies with engine RPM and throttle position. But, there exists some pressure differential at this point to operate the recirculator.
Having received a partial vacuum from the air box, the recirculator device then distributes this vacuum to the engine crankcase and the oil tank. Why? Again to recover vapors, push them toward the engine inlet, and allow the engine to burn them, but also to lower the pressure in the system to promote "boiling" off the water vapor within the crankcase and oil tank system. The chocolate milk and/or brown mayo found in the system due to emulsified oil/water, is more easily extracted with both heat and lower pressures applied to it. There is also a possible benefit of helping the rings draw down on the piston lands to seal the combustion chamber and reduce blow by gasses. Some race engines actually add and use a vacuum pump to increase the ring seal effectiveness.
For the SOHC4 system to operate correctly, the recirculator should not have any open drain tubes to outside atmospheric pressure. Even the lower water drain tube should be capped and/or severely restricted to incoming air flow, in order for the small vacuum present at the top of the air box to reach the crankcase and oil reservoir.
I hope this helps proper understanding.
Cheers,
