Topic: Hose routing for CB750

[78whiteorbs]

Got a 73 ,just got it . After  short ride I noticed an Exxon Valdes Oil leak that was only present after riding. Hosed my rear tire and such. Fond that it was coming from lower  tank spigot connection. Breather from up above the carbs was routed to this lower spigot and that is were the oil was coming from . This doesnt seem right at all . I put a small hose clamp around that hose and the leak stopped, seems it was either coming  from that beather tube or the spigot.  There is one more hose hangin with a bolt inside f it (I am thinking that aint stock )  Any how I road to the store and back and there was some vapor/smoke coming from the oil tank .

I  am thinking that the breather should not be connected to that spigot ,right? Also the lower crankcase hose  behind the  transmission   is just hanging with nothing coming from it . A breather  hose should just be connected to a filter or something right?  Anyone have picture of where those hoses go?

[Hinomaru]

Sounds like your routing is all hosed up.   For your ’73 CB750 K:

The oil line from the rear of the crankcase connects to the rear oil tank fitting. (red circles)

The upper oil tank breather hose (blue arrow) routes downward and vents to the atmosphere.

The valve cover breather hose (blue arrow) also vents to the atmosphere. 

[78whiteorbs]

Thanks for the detailed explanation and pix-

Seems the vent tube was the one with a bolt in it for some reason which would explain why the pressure was kicking oil out of the lower spigot ,or?

When I pulled the breather hose off of the lower spigot I got some oil from the breather tube that emptied but I would suppose it's just what shot up from that lower back spigot into the hose. If that spigot is left open pressure  that isnt vented from the tank would cause a massive oil leak right?

Any reason you could think someone would cap off a vent? Only thing I can think of is that it was blowing oil out the vent for some reason. Also there is no oil coming from the crank case (red circle) is it suppose to be coiming out of here or going into here because there is nothing exiting the tube.

[MCRider]

On modern bikes, breathers sometimes have caps and are meant to be drained of their condensation occasionally.

The lower vent from the crankcase does connect to the lower spigot on the tank as mentioned. A very common mistake is to leave it open. And it will over time blow oil out onto the tire. But it was many years before i was educated by someone here on the forum as to the actual workings of that vent.

The lower spigot on the tank actually enters the tank as a pipe which is then routed up to the top condensing knob at the top of the tank. In other words, condensing oil in that top knob runs DOWN the pipe (look in an empty tank you'll see it) and out the spigot, into the hose, into the rear of the crankcase, where it drips ona small sheetmetal pan, which then directs the oil flow into the primary drive shaft and out the CHAIN OILER! If you ever saw a crankcase apart, you'd have an AH HA moment.

But if the hose is gone or disconnected oil (vapors mostly) will reverse and come out the tranny. I suppose with it connected you may actually have a 2 way street, vapors rising to the knob on the tank and returning as oil for the oiler. anyway the oil in that lower hose never touches the oil in the tank. It travels in a tube inside the tank.

And since there is very little oil in the transmission itself, if you were to leave that lower hose off the back of the transmission and rev it up, probably nothing would come out, as you've noticed. Its only after riding it around for a while that it will puke all over your tire.

[78whiteorbs]

I see- so that would explain why my "chain oiler" isnt oiling 

thanks for that explanation- makes good sense- 

[MCRider]

I see- so that would explain why my "chain oiler" isnt oiling 

thanks for that explanation- makes good sense-

Yup!

[mycb750k6]

So if that breather 1/4" OD  hose that goes from the bottom rear of the crankcase up the the oil take were to get cracked or had a hole in it, would I see oil fumes coming from that area. Because I do and the oil fumes do not come from any place else except down by the crankcase spigot so I'm assuming there is some amount of oil movement in that hose and under some amount of pressure???

[MCRider]

Lastly first, there is no pressure, other than gravity.  Liquid oil will flow from the tank, the condensor bulb up high, thru a metal tube inside the tank, to the spigot you see near the lower backsideof the tank where the rubber tube hooks up.  Then into the crankcase where it drips on the little catch plate that routes it into the final drive shaft and out the chain oiler. In the other direction you could get vapors. The line is never full of oil so there is room for the 2 to coexist.

As many threads here, and my personal experience will attest, if you leave that tube off altogether eventually a significant amount of oil will exit the transmission onto the tire, usually from repeated accel/decel cycles where the oil in the trans is getting sloshed around a lot and makes its way out that tube. So fix it, soon.   

[Floshenbarnical]

Sorry to resurrect an ancient thread

My cognito moto oil tank does not have the connection for the oil breather hose that attaches to the spigot on the back of the crank case

Should I attach a hose to this, run it up vertically a little ways, and cap it off? My theory is that a little length of hose will allow the vapors to collect and drip oil back down the hose into the crank case

[willbird]

I learned the hard way about using a plain old rubber automotive hose for that vent line.

I re assembled my bike from a frame up back in early 1990's and just used some generic automotive rubber hose the right size.

Once I had ridden the bike a few hundred miles that hose got all gummy, melted, and weird and sprayed oil all over the rear tire and everything else in that general area.

I replaced it with an Earls econobraid hose and it has worked great for the 30 years since :-).

[Floshenbarnical]

I learned the hard way about using a plain old rubber automotive hose for that vent line.

I re assembled my bike from a frame up back in early 1990's and just used some generic automotive rubber hose the right size.

Once I had ridden the bike a few hundred miles that hose got all gummy, melted, and weird and sprayed oil all over the rear tire and everything else in that general area.

I replaced it with an Earls econobraid hose and it has worked great for the 30 years since :-).

So where did you run that vent line to from the spigot? Did you cap it and call it a day?

[HondaMan]

If you don't have that spigot on the oil tank, run the new hose up from the back of the engine into a tee with the one from the top of the engine, and then run the resulting "joined" hose down the right side of the frame, turn near the swingarm, and go back to stop it just before the rear axle. This was how Honda set up the 750K6 and later versions when the oil tank was the type used on the K7/8 bikes. If you can, go to either South Sound Honda's parts fiche for the CB750K6 or to CMSNL's site for the CB750K6 or K7/8 to see the pictures in their parts diagrams, that might help, too.

About the fuel-feed troubles in the other post: make sure the hose from the tank's petcock only goes downward, or briefly horizontal, before entering the carb's fuel feed line. If it goes below the fuel feed and back up it will develop a hydraulic lock when the air temperature behind the engine's cylinders gets hot - this is caused by thermal expansion of the gas, coming out of a cool gas tank where the wind is passing by, and then trying to enter hot carbs. It is literally a 'bubble', pushing back at the gas petcock line.

[willbird]

I learned the hard way about using a plain old rubber automotive hose for that vent line.

I re assembled my bike from a frame up back in early 1990's and just used some generic automotive rubber hose the right size.

Once I had ridden the bike a few hundred miles that hose got all gummy, melted, and weird and sprayed oil all over the rear tire and everything else in that general area.

I replaced it with an Earls econobraid hose and it has worked great for the 30 years since :-).

So where did you run that vent line to from the spigot? Did you cap it and call it a day?

I hooked it up where the OEM intended it to go. My failure was using a type of hose not intended for hot oil and vapor