Topic: CB750: oil from tank overflow - causes?

[MCRider]

I wasn't having any problems with oiling, at drag strip or Dyno runs or during the brief 160 mile break-in. Several oil changes, more than 4 probably. Always put 3.5 qts in.

So, we take it to the Ohio Mile. While sitting on the start line, engine running, it puked a little, according to witness, I wasn't there. Rick took the run anyway, slightly more than 1 mile at top speed of 125,and pulled back into the pits. I hadn't seen it up to this time. He turned it off and parked it. In my post run exam, I didn't notice any oil on the tire. Shortly, on the side stand, it leaked out the tank overflow, about several tablespoons. Then it stopped.

I dbl checked oil quantity. I had used a 4 liter bottle of BelRay for the pre-run oil change. So I measured what was left in the bottle. nearly exactly 1/2 liter. Meaning the tank was not overfilled. Is there something I don't know about that can cause oil to leave the tank overflow?

PS: OK, I just went and looked it up in my Clymer. It says oil capacity = 3 qts or 2.7l.  If that's correct, I guess I have been overfilling it?

PPS: My second Clymer (different pub year) also says 3 Qts. My K2 Owners Manual says 3.7Qts. That must be with a dry engine.

[flybox1]

What do you have connected to your rear lower case vent ?

[MCRider]

What do you have connected to your rear lower case vent ?

The rear lower case vent is attached to the lower spigot on the oil tank. Good question, but I'm aware of that one.

[flybox1]

What do you have connected to your rear lower case vent ?

The rear lower case vent is attached to the lower spigot on the oil tank. Good question, but I'm aware of that one.

The lower spigot is the tank overflow, but if it was attached, how could it leak?   
Not understanding where the oil came from 

[MCRider]

What do you have connected to your rear lower case vent ?

The rear lower case vent is attached to the lower spigot on the oil tank. Good question, but I'm aware of that one.

The lower spigot is the tank overflow, but if it was attached, how could it leak?   
Not understanding where the oil came from 

I's a K2 and the oil came from the upper hose that comes out of the bulb/condenser thingie at the top of the tank and is vented to the atmosphere. I had rerouted it to a breather mounted on the drive sprocket cover. So the oil dripped on the cover and down the side stand, missing the rear tire.

[ekpent]

 May get flamed but back in the day I would just dump in 4 full quarts during an oil change and never had any overflow problems rather then leaving a little in a bottle. My 1974 750  right now shows it over the 'full' line on the stick, no problems running hard on my recent trip.

[MCRider]

May get flamed but back in the day I would just dump in 4 full quarts during an oil change and never had any overflow problems rather then leaving a little in a bottle. My 1974 750  right now shows it over the 'full' line on the stick, no problems running hard on my recent trip.

That's been my experience also, though I kept it at 3.5qts. Ran it as hard as I wanted, never had a drip. Figured as long as there was nothing on the floor and it didn't smoke, no need to check it between fills.

Now this?

Is there something about the functioning of the condenser bulb on the tank that could be causing this now? Even though its not been leaking/overflowing till the Ohio Mile.

[scottly]

I's a K2 and the oil came from the upper hose that comes out of the bulb/condenser thingie at the top of the tank and is vented to the atmosphere. I had rerouted it to a breather mounted on the drive sprocket cover. So the oil dripped on the cover and down the side stand, missing the rear tire.

You lost me here. First of all, there is no such thing as an overflow. The oil/air separator inside the tank drains through the spigot on the back of the case. Is this the hose you rerouted?

[MCRider]

I's a K2 and the oil came from the upper hose that comes out of the bulb/condenser thingie at the top of the tank and is vented to the atmosphere. I had rerouted it to a breather mounted on the drive sprocket cover. So the oil dripped on the cover and down the side stand, missing the rear tire.

You lost me here. First of all, there is no such thing as an overflow. The oil/air separator inside the tank drains through the spigot on the back of the case. Is this the hose you rerouted?

The K2 separator is outside the tank. There is a large diameter hose, same size as the hose off the cam cover, that is just open to the atmosphere, routes down to the swingarm and hangs.. It's from this hose that the oil came.

There is a smaller spigot down lower that connects to the one on the cases. It connects to the air/oil separator inside/through the tank as a solid metal tube.

Background: I've lived with this bike since 1974. Had it apart many times and know where the lines go. But having oil come out of that top one is a new condition to me.

[scottly]

What did the dip-stick show?

[MCRider]

What did the dip-stick show?

I just checked and its slightly below the full mark. Same place it was before the event.

I didn't mean to sound cocky in my last post, just providing perspective. I've never had that much oil, a few tablespoons I'd say, come out of that hose. It acted as an overflow but I guess its really a breather, more accurately. Appreciate your comments.

[scottly]

OK, after thinking about it, I have a theory: the 125 MPH 10,000 RPM run produced so much foamed up oil froth from the scavenge pump that it overwhelmed the separator's capacity and filled the tank, and the froth condensed in the breather tube. The simple fix, if this is the cause, would be to route the vent hose to a catch-can. Most racing venues require catch cans; I'm surprised they didn't at the Mile?

[MCRider]

OK, after thinking about it, I have a theory: the 125 MPH 10,000 RPM run produced so much foamed up oil froth from the scavenge pump that it overwhelmed the separator's capacity and filled the tank, and the froth condensed in the breather tube. The simple fix, if this is the cause, would be to route the vent hose to a catch-can. Most racing venues require catch cans; I'm surprised they didn't at the Mile?

No, they don't require catch cans and were a little light on the safety wire, too. But I don't think they get many vehicles with open air vent tubes. We used catch cans in WERA back in the late 70s when I road raced briefly. I can do that. But I was really concerned with why. I think you may have it figured. I tried to think what all I had changed. I changed from Castrol 20w-50 to BelRay 20W-50. MAybe the Castrol doesn't froth as much. Also, the drags are over before the WFO mile begins.

Do you think there was a malfunction of anything? Or just doing their jobs? I'll use the BelRay I've bought, up in the upcoming Dyno runs and Drags. Then for the next mile, Rick wants me to switch to synthetic so maybe that won't froth as much.

[scottly]

I don't think there is anything malfunctioning, you are just pushing the system to the limit.

[Henning]

Can be caused by too much crankcase pressure, if for example the valve cover breather is pinched or blocked, or if the head gasket is weeping pressure into the engine. It's an uncommon but known problem. There are a few threads about it. I followed your build thread so I know what you've done to the bike.

[MCRider]

Can be caused by too much crankcase pressure, if for example the valve cover breather is pinched or blocked, or if the head gasket is weeping pressure into the engine. It's an uncommon but known problem. There are a few threads about it. I followed your build thread so I know what you've done to the bike.

I'll check the valve cover breather as it was rerouted before the last event as well.

Thanks for watching!   

[MCRider]

I don't think there is anything malfunctioning, you are just pushing the system to the limit.

Thanks!