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Oil blowing from the oil tank vent

Bones

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Pefferlaw Ontario Canada
Just found the forum while trying to get idea's on whats causing a major oil leak from my buddies 74 CB750 SOHC4. I also have a 1971 which I am in the process of Caffing, anyway if anyone has any advice on the cause of this leak I would be grateful if you could share it.
Problem:- Oil is blowing out of the oil tank breather hose in large quantities (confirmed that this is the source of the leak).
The oil level is correct and will leak until no oil is left. The oil and vent lines are hooked up correctly and was running fine like this last year. The top / rocker cover vent line is open and vents well.
I do not know about the rear line / gearbox line and have not inspected it.

Any help would be apprciated.
Terry
 
Hmm.. that sounds interesting.. All I can think of is a faulty check valve so the oil is draining from the tank to the engine sump. If you keep adding oil, the system is overfilled and it will flow out the breather, then the oil in the tank will drain back into the engine making you think it's empty. Does this happen after a fresh oil change (drain the tank then sump, then fill it with 3.7 qts of oil)?

By the way, welcome to the site! We'd love to see some pictures of the bikes! :)
 
Thanks for the reply, this occurs no matter the oil level. We have drained all the oil and put in the exact amount and she will pump her out of the tank breather until no oil is showing on the stick.
I havnt done a compression check as my gauge is loaned out! ( forgot who borrowed it ) `but the cycle is running so smooth.
I will post pics.
 
Ok, In the oil tank at the top there is an oil/air separator There are two hoses coming out of it near the top of the tank. The big one is the vent to atmosphere and shouldn't be hooked up to anything. The other smaller one should go to the back of the gear box, this is the line that the oil is supposed to return to the engine when it accumulates in the oil/air separator. If the small line is blocked it wont let oil drain out of the separator. This is how it is from factory. The problem is I dont know how you are loosing large amounts of oil from the breather tube because the only way oil could actually run out the breather tube is if the tank was full to the top. Unless you have huge amounts of blow by, because thats what the oil/air separator is used for. I would carefully check the line from the tank to the gearbox and make absolutely sure it is open going in the air box and into the oil tank, I would use a little bit of compressed air and blow both ways. I will think on it and see if I can come up with something else but this would be what I would try first.
 
Thanks digger I did check that out and it seemed fine , what do you think about this ?

Okay I maybe talking out of my ass here as I have little experience with the SOHC engine but I did manage to get a crummy Haynes manual that had a crap picture in it showing the oil circuit. Maybe someone with better knowledge could comment on my conclusions.

At the back of the engine is a line that leads upto the oil tank breather assembly and as you told mentioned it is to drain any oil that enters the internaloil tank breather back to the engine.
Now looking at the diagram oil is supplied to the engine via the main pump and to the gearbox by the scavenge pump. The scavenge pump shows a tee one to supply the gearbox and the other sending oil back to the tank.
Now lets say the return line from the scavenge pump to tank gets restricted then the main flow will go to the gearsbox , if the restriction is bad enough all the oil could go to the gears then presssurising the box and eventually sending oil back up the rear engine breather drain tube and out the breather.

A lost in space mechanic.
 
Yes you are correct in how the oiling works in the 750. The oiling passage to the transmission has a small restictor in it so all the oil doesn't just go to the transmission, it is about 1/4" in diameter maybe little smaller as opposed to the oil return line which is 1/2". That restriction in the transmission forces the rest to be returned to the tank. The oil in the trans then runs back to the engine sump where it is picked up by the scavange pump and the precess starts over. Anyway, now have you check the main oil return line? I guess it could be possible the rubber inside the line has collapsed.? You should also be able to see oil being returned to the oil tank. Not a lot at idle but if you bring the rpm up a bit it should flow in the tank pretty good, it dumps out near the top of the oil tank and cant see the end of the tube but you should see oil circulating and splashing around, the 750's return a fair amount to the tank. You could check and see if oil is being pumped out of the breather drain tube. Another though would be to remove the main oil return line from the tank start the bike and see if oil is being pumped out, be sure to keep oil in the tank though so you are oiling the motor. If you do this do with caution so you dont run it out of oil.
 
Had a similar problem years ago with my ford ranger 4 cyl, oil was blowing out the dipstick and PCV valve!, turned out it was nothing a ring job wouldnt fix, and it did! . . . go figure. I guess the pressure going past the rings into the crankcase was causing the havoc. It may not be your problem, but after my ferd experience, oil blowing out of any vent would now lead me to do a compression test sooner than later
 
So we didnt have much time to spend on the Honda this weekend as it was so nice we had to go for a spin. We did however check the compresion again and found it good as before. I then took the line off the rear of the engine and installed a clear plastic one which I dropped into the tank filler cap the other end I plugged. My thoughts were that oil would blow back from the engine from the rear line but nothing came out and still the tank vent dumped a ton of oil during a 2 mile run.
The oil is not frothing or bubbling badly so my thoughts are that we have a problem internally with the oil tank were the oil is entering the vent chamber when it shouldnt.
Beer took over then as we plan to try another oil tank which we have but needs a good wash.
I will post again.
 
Sounds like a good next step to me. If you got another tank give it try at least you will either fix it or eliminate that as being a problem. Keep us posted.
 
RESULT!
We finally got to the bottom of this mystery! we changed the oil tank and away she went. We just finished a 100 mile trip with not a drop of oil showing on my buddies 74 CB750 so all is well. I do plan to cut the tank open and see whats going on inside but that will be a winter project.
He's a happy camper and now I can address my own oil leak issue :(
 
I am really curious as to what went wrong inside that tank. Glad you got it figured out, nothing better than the feeling of accomplishment when you figure out a pain in the **s problem. A tube must have either rusted out or cracked inside the tank. Anyway keep us posted!
 
Let me know if u figure it out, I'm having the same problem and just bought this bike..... New to this whole thing and trying to find "how to" things on the net is harder than I expected.... Thx '76 CB
 
The fix was a different oil tank. There has to be a break in the air/oil separator inside the tank. Swany, tell us a little bit about your problem. It may be like Bones problem but I have never heard of the problem they had with there tank so I would bet it is something else. But you never know, two bad oil tanks could happen. We can help with any questions or problems you may have, lot of people on here have some good ideas.
 
Heres a quick run down of what happened.
1) Oil was blowing from the oil tank vent on any run more than a mile. Oil would not come out when revving in the driveway. half the oil in the engine would be lost on a two mile run.
2) We checked the engine breather and no oil was coming from there nor was any blow by when revving.
3) We did a compression test twice all was good we did not do a leak down.
4) We blew (by mouth to see if there was any restriction) down the drain line from the oil seperater to rear engine , Both ways were free, we shot compressed air down both to ensure open passages.
5) We ran the bike with an open oil cap.
None of the above did anything to help.

We changed the oil tank and the problem was solved. We will open the tank up one of these days, we are just busy catching up on the miles we lost.
 
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