Topic: CB750 oil tank/valve cover venting

[Reindeer]

I've got a 75 CB750 that was modified by the previous owner and I'm going back and redoing some of the routing of hoses and wiring. The bike has a hose that connects the back of the valve cover to the top of the oil tank, and between has a hose t'd off that just runs down and hangs open ended. Is this how its supposed to be? Does it need to be vented to the open air? Is it more to vent the oil tank or the engine, or both?

[immortal]

I was just working on the same thing on my bike. From what I understand and I'm sure I will be corrected if I'm wrong. The hose from the valve cover goes along the top of the frame and drops down by the swing arm to vent. The hose from the oil tank should plug into the rear of the crankcase, if oil overflows and or expands in the oil tank it would flow though that tube into your crankcase so there are no spills etc. The hose from my valve cover I dropped down and added a nice little washable filter that attaches to my rear engine bracket. I hear if you do not route that Oil tank hose to your crankcase and just leave it dangling there could be oil splutter that makes a mess and possibly even splutter oil onto your rear tire if it is positioned badly.

[Elan]

...Is this how its supposed to be? Does it need to be vented to the open air? Is it more to vent the oil tank or the engine, or both?

Thats not how it is set up stock, but seems setup correct. I think earlier models had a little airbox inlet so it vented into the airbox. Essentually both those lines need open air, so your PO just connected them and ran the open air line down to have one less hose going down.  Dont connect them together with no open air like I did, you engine will get angry.

[Reindeer]

So if I understand both the port on the back of the valve cover and the oil tank just need to be vented to open air, and not necessarily to each other?

[bikerbart]

correct.the one that needs to go to the bottom of the oil tank is the crankcase breather,bottom rear near the swingarm.The other two vent to atmosphere