Topic: 1971 CB 750

[Bodain]

Haven't been on here in a very long time. In past years I've worked numerous old CB 750. Mostly 74-78....
I now have my hands on a 1971.. Things are a little different. At any rate... I've got a quirk, with my oil tank. If bike is running on center stand. No problem. The first time I rode it. I observe that I'm leaking oil. More study to figure out where it's coming from. I put a clear hose on the port on the bottom inside of oil tank. It runs freely out there. Looked over shop manual with no clue...
Is someone going to tell me there is supposed to be a line between there and the port on back of engine??? Oil flows that route from the tank?

[Terry in Australia]

Yeah mate, there's supposed to be a hose between the spigott at the bottom of your oil tank, and the one at the back of your gearbox, same as the ones on your other CB750's, they all had them. Cheers, Terry.

[BobR]

Yeah mate, there's supposed to be a hose between the spigott at the bottom of your oil tank, and the one at the back of your gearbox, same as the ones on your other CB750's, they all had them. Cheers, Terry.

 Interesting! That small spigot on tank of my K4 has been blocked off since I got the bike 20 years ago. The larger vent hose on top is connected at the back of the gearbox though. Hasn't seemed detrimental as Ive put a few thousand miles on this way but now I'm wondering....

Edit: Did a little digging.  Yeah the spigot in question connects to that rear engine breather. The spigot itself isnt at oil level though.It is connected internally to a pipe that rises above oil tank level and terminates in a "condensation" / collection bulb. Any oil that collects in it goes down to that spigot, runs into the engine breather and supposedly collects on a small plate for the chain oiler. If thats the case my oil tank breather connected to it with this spogot blocked off is ok. I dont want the chain oiler anyways. Make sense? Bob

[Terry in Australia]

Yeah Bob, that's right. I argued with a bloke that it can't be for the chain oiler because they still had them on the post K6 bikes that had no chain oilers, but who knows? Interestingly they increased the size of both spigots on the later non-chain oiler models, so I've always maintained that it's just a crankcase breather, but who knows?

If I was you I'd re-route the hoses though, The valve cover breather hose just vents to the atmosphere, and the top breather hose from the oil tank should vent to the atmosphere too. If you've got something venting into your oil tank, you need some way for it to vent out. Cheers, Terry.  

[PeWe]

Honda made a mistake on the K6 oil tanks. Lower spigot connected to case as before, BUT NO ventilation from tank to let the overpressure out. This will cause crank case oil weeps, especially with more displacement pumping the engine. I should have understood this for at least 32 years ago....but the coin did not fall down until earlier THIS year!

I have done a T-connection on the breather hose between case and tank, one hose going upp 10" then down again breathing beside the valve cover breather hose. I get some oil spatter coming out so I need to make a small oil catch can with a pipe or hose going to front sprocket as automatic chain oiling system.

OR, I can also use an earlier K2 oil tank I have as the photo here with ventilation on top. Need to connect the thicker K6 hose to the smaller connection on tank. 13 mm hose to 8 mm or so. Swap tanks might be easier. The other tank has also a connection for oil temp sensor which can be good.

[PeWe]

One more thing.. My K6 have the chain oiler. CB750E-24694xx
I could not understand where the oil drops came from until I remembered the oiler that I switched off for some decades ago. I missed to tighten the screw at last rebuild.

[ekpent]

 I do not believe that the K6, at least in the US came with the chain oiler.

[BobR]

Honda made a mistake on the K6 oil tanks. Lower spigot connected to case as before, BUT NO ventilation from tank to let the overpressure out. This will cause crank case oil weeps, especially with more displacement pumping the engine. I should have understood this for at least 32 years ago....but the coin did not fall down until earlier THIS year!

I have done a T-connection on the breather hose between case and tank, one hose going upp 10" then down again breathing beside the valve cover breather hose. I get some oil spatter coming out so I need to make a small oil catch can with a pipe or hose going to front sprocket as automatic chain oiling system.

OR, I can also use an earlier K2 oil tank I have as the photo here with ventilation on top. Need to connect the thicker K6 hose to the smaller connection on tank. 13 mm hose to 8 mm or so. Swap tanks might be easier. The other tank has also a connection for oil temp sensor which can be good.

 Essentially the PO on my bike eliminated the rear breather on my K4 by connecting the upper tank vent directly to it (the rear engine breather) while blocking off the smaller internal tank one on the side of the tank. Im replacing the hose this winter so I may add a T to this set up. Bob

[PeWe]

It must be better to connect the hoses as designed. Case breather to lower spigot on oil tank.
Upper spigot on oil tank get 1m hose routed parallel with valve cover breather hose?

[BobR]

It must be better to connect the hoses as designed. Case breather to lower spigot on oil tank.
Upper spigot on oil tank get 1m hose routed parallel with valve cover breather hose?

   Yeah good point. Wondering why someone decided to hook it up like it is now. Ive only put about 4K miles on it in the 20 years Ive owned it but it hasnt been an issue.... Now I realized my rear engine breather is a larger diameter than it should be for a K4...Bob