Topic: cb 750 f2 oil issue

[Mafia]

Have a custom oil tank on  my bike that's causing me some issues. It seems that the oil lines are leaking. Not from the connections themselves but from the hoses. Anyone ever have this problem? Did the stock oil tank have a way to vent some pressure?  It's the last thing I need to do to my bike before I can ride. Any help would be appreciated.

[70CB750]

I saw your post yesterday and lost it again 

Oil lines on CB750 SOHC carry zero pressure, one supplies oil to the oil pump and one carries oil back to the oil tank.

The only time I had a leak was when the hose was not tight enough.

You have a custom tank, are your hoses custom too? 

[madmtnmotors]

Did the stock oil tank have a way to vent some pressure?

Yes.

[calj737]

Oil lines on CB750 SOHC carry zero pressure, one supplies oil to the oil pump and one carries oil back to the oil tank.

70CB - pardon me, as I am not a 750 guy, but this statement seems incorrect. The oil lines are plumbed into the motor which is under pressure, thus moving the oil up to the tank. It could only do this if under pressure, no? If I am not mistaken, the original tanks had a vent to alleviate the pressure build up from the running motor.

I've not seen a gravity return to the oil tank, so the feed and return must be under pressure. Or am I missing something?

[70CB750]

OK  , I give you that, let's say almost zero pressure. 

The oil return to the tank goes into the bottom of the tank - means it goes against the pressure of 6? or so  inches of oil.

While ago I was thinking about hooking up a gauge to measure this

The other hose gravity feeds the oil pump - again no big pressure to be concerned about.  You can use a hose and clamp and it will hold the oil just fine.

Please, look at the oil schema, it will give you better understanding than my clumsy English.

I think Mafia has a point, custom tank could be missing a vent.

[calj737]

 I wasn't trying to nit-pick with you. I suspected your written words were betrayed by the language barrier. Agreed, gravity fed systems have only a negible pressure. Perhaps best described as not "mechanically pressurized". I also suspect the absence of a vent is causing the oil to leech through the lines.

Thanks for clearing me up on that.

[Mafia]

 No not custom lines. The  fittings were removed from the stock tank and welded in place on mine.when I get home I'm gonna loosen the fill cap on the tank and fire it up. No leak well it needs a vent. Absolutely have to solve this problem. This is the first time this bike has ran in 27 years and I simply must ride it.

[70CB750]

I wasn't trying to nit-pick with you.

It did not come across as such, the post needed more clarification.

[MCRider]

I wasn't trying to nit-pick with you.

It did not come across as such, the post needed more clarification.

And confirmation. The pressure exists mainly between the pump and bearings, main and cam. Other than that its a gravity drain to the sump, where the pump picks it up again and fires it back to the tank, under pressure but no resistance. Secondarily is pressure to the trans and gravity to sump.

The stock tank cap/diipstick is a vented type. Your cap appears to seal tightly? The stock tank also has a vent to the atmosphere from the top of the tank. Without those, you could be building some pressure in the tank which would constipate the whole system. Maybe. OCICBW

[Mafia]

Well the cap/dipstick on the new tank was from the stock tank. I think we may be on to something when it comes to venting this tank because it is indeed sealed. Just pray that's the answer. A lot of hours in the custom tank. I just don't want to put any holes in it only to find out that oil will come shooting out.

[MCRider]

Well the cap/dipstick on the new tank was from the stock tank. I think we may be on to something when it comes to venting this tank because it is indeed sealed. Just pray that's the answer. A lot of hours in the custom tank. I just don't want to put any holes in it only to find out that oil will come shooting out.

Oh my bad, you have an F2 whihc is a different cap than the K2 I'm familiar with. Is the F2 cap unvented?. In the K2, and assumed the F2, the venting is for fumes, heat expansion, nominal pressure changes. In a perfect world there is very little pressure in the tank itself.

[MCRider]

The K2 tank vent was a large (3/8"?) hose from the top of the tank just laying out in the open above the swingarm. The F2 was probably routed into the airbox for emissions control. Vented nevertheless.

[MCRider]

I see on the tank in the oil diagram, there is a small round "catchcan" affair, that has a spigot on it. A vent hose would attach to that and hang to the ground or more likely into the airbox...which you no longer have.

NEat oil tank by the way.

[Mafia]

Built the tank using the guts from  a stock tank. I put a raised vent tube in the tank. It vents out of the rubber tube on the bottom. I put a pod filter on the crank breather. You would think that would be enough venting. Might be just the oil lines after all. I was under the impression that failure in those lines was highly unlikely.

[Mafia]

Another shot of the tank. Only one I had here at work.

[madmtnmotors]

The original F (0/1/2/3) did not have a vented cap, but did have a vented tank. The F's (0/1/2/3) along with the K7/K8 and I believe the A models as well had a vent connection at the bottom of the oil tank that was basically a stand pipe inside the tank that extended up above the oil level and was connected to the back of the crankcase behind the transmission. Search for an oil tank cutaway, I have seen more than one.

Better yet:

http://lmgtfy.com/?q=cb750+oil+tank+cutaway

The first result listed has a picture in the thread....

[70CB750]

Hmm,  I was wrong, the return does not go to the tank bottom, learning something new everyday 

I believe on the K0 the rear end engine vent is connected to the pipe called OVRFLW here.

[MCRider]

Built the tank using the guts from  a stock tank. I put a raised vent tube in the tank. It vents out of the rubber tube on the bottom. I put a pod filter on the crank breather. You would think that would be enough venting. Might be just the oil lines after all. I was under the impression that failure in those lines was highly unlikely.

Does the tank actually vent to the outside somewhere, I missed that.

The nipple on the trans does hook to the standpipe on the tank, which travels up the inside of the tank to what used to be that catchcan on your diagram, then to the atmosphere.

[Mafia]

I don't believe it does vent to atmosphere.  I'll have to check when I get home.

[madmtnmotors]

Does the tank actually vent to the outside somewhere, I missed that.

The nipple on the trans does hook to the standpipe on the tank, which travels up the inside of the tank to what used to be that catchcan on your diagram, then to the atmosphere.

The vent tube connection to the back of the crankcase, behind the transmission, from the bottom of the stock oil tank, is your indirect connection to the crankcase vent at the back of the valve cover... as long as you have the vent connection to the back of the crankcase from the vent pipe in the tank, then you are vented. Some picts or detailed diagram of you "custom" setup might help alleviate some of these "stabs in the dark"...

[MCRider]

I don't believe it does vent to atmosphere.  I'll have to check when I get home.

It may not have anything to do with your leaky lines, but...

I've often wondered about the round chpper tanks, etc that I've seen, how close they are to stock plumbing and still work.

[Mafia]

Well my tank IS stock plumbing. So I can't imagine how those round ones fair. Carpy knows what he's doing though. It just has to be the lines. Gonna loosen the fill cap tonight and fire it up, see if it leaks. If it still does then venting is not an issue (fingers crossed) and it the lines themselves.

[koendd]

If had the same problem with my F2 but mine had nothing to do with venting since I use the original oil tank.
Rebuild my bike few months ago, rode about 200miles and then all of a sudden small oil puddle under the oil tank.

Took it out and had a hole in the oil return hose.
No idea if it was there before but had the tank off during rebuild and cleaned the lines, repainted the connections and didn't see any damage at that time.

So just took it to a shop over here who put new hoses on them for about 18$

[Mafia]

So where is the best place to get both hoses if it comes to that?

[70CB750]

Here in the clasified section.