Topic: CB750 oil tank vent tube

[Turbogrimace]

How important is this little portal of neutral pressure?  I'm making an earl tank and the vent tube will be at the top but can still potentially get splashed with oil.  I dont see how its too much of a problem because i'm going to connect it to the rear of the engine as is standard.  Just curious if anyone has experienced any problems with oil filling the vent tube in an oil tank.

Thanks for the info!

[Old Scrambler]

Hooking two vents together will be a problem!  The rear vent hose from the tranny will actually flow oil at high RPMS.  That is why it is linked to the oil tank at the filler level. The top vent is to atmosphere.  If your filler-cap is well vented and tall..........you may not need the top vent.

[Hinomaru]

One solution is to add a simple oil-air separator in the oil tank vent tube fitting and/or vent tube. That way any splashed oil can drain back to the tank instead of going out the vent tube. Standard on the Harley XR-750 and select production street bikes. Sturdy plastic part that can easily modified to suit your needs and around 5 bucks each from a H-D dealer. Part number: 25329-91 

[Turbogrimace]

My rear tranny vent is hooked to my oil bag vent on my old long bike.  It connects to a tube that sticks up in a little nubbin at the top of the oil tank above the oil level.  There's no issues with connecting them and the pressure in the tank is relieved through the tranny vent into the engine.  I havent had a problem.

On that bike, however, the line is much higher than the oil level so it would be pretty hard to get oil down into the vent tube from the oil tank.  My new tank is vertical, and small (2.5 qts max) and the vent tube is only about 1 to 1.5" above the oil level at the front, so under heavy braking or a dope endo I could potentially flow oil into the vent line and down the engine.  My guess is that the pressure from the tank (if any built up at that time) would push the oil down into the engine as long as the pressure required to pop the oil cap off was greater than the pressure to push the oil down. 

I think I'm just thinking too much at this point, but curiosity always gets me.

Hinomaru, I'm not sure how those separators work unless the vent tube faces downward into the tank where they are installed, but thanks for the info.