High rpm, crankcase pressure, oil leaking

[Guest]

I am having a problem with oil leakage.
It was coming from the base gasket, so I pulled the top end and redid the gaskets. So no leaking from there, now its coming from the shifter shaft
It seems to only be leaking when I have run it at higher rpm like 6k+
I thought it was because of high oil pressure. But now I am thinking its crankcase pressure forcing the oil out. I plan on removing the valve cover and opening up the breathing passages.
I think I have read about this being a problem with overbored bikes somewhere.
The top end is fairly new so its not sealed yet, causing blowby?

[Steel Toed Tank]

I run 95% at 5,000rpm + and I don't get leakage, but I can see seepage around some of the gaskets. I would be interested in an answer to this as well.
I have no idea how many of the gaskets are OEM or have been replaces though.


Tank

[Steve]

The only way that pressure is going to build up is if your breather port at the top of the valve cover is blocked. However, that is a relatively large port and tube leading away from the engine, so it's rather unlikely.

You say your bike is overbored. Any chance you also have pods on it? If so, how is your breather hose routed?

.

[gearhead13]

I have never run a hose off of the breather, no oil ever came out except when I didn't have the steel mesh in there.
I pulled the valve cover and top and opened up the holes to allow the pressure to escape. The stock breather was designed for 1074cc, mine is 1327cc. This only started when I went to an 80mm bore.

[Steve]

Would you believe they use the same size breather on all the GSes from 300 to 1150?

The only source of pressure in your crankcase is blowby that comes past the piston rings. That engine should have been broken in by now, and should not be leaking that much pressure into the crankcase.

.

[7981GS]

Shifter shaft seals leak from time to time.
Replace it.

Eric

[OldVet66]

Air is going in and out of the breather tube with the rise and fall of the underside of the pistons, and it is not completely canceled out by the configuration of an inline four. It is my understanding that at high rpm, there is a point where the vent is too small to accommodate this and it effectively creates an air dam that allows a positive build up of pressure in the crank case adding to blow by and seal leakage if the seals are marginal. That is why I initially installed a Hayden Krank vent. It allows the pressure to go out but meters a small return to maintain a 3 to 5 lb. vacuum in the crank case. I have watched a Harley (early model Evolution) stop leaking oil to barely seeping as soon as the Hayden vent is put on the vent tube. I also discovered some other added benefits for the inline four, better low end power and better gas mileage, but no one on the forum seems to want to hear it. My Hayden valve came with a defective umbrella valve that failed stuck in the closed position. It puked oil out of my shifter seal and smogged the neighborhood, so I also know what a positive pressure in the crank case will do if the vent is blocked. That was an extreme event before the factory fixed the problem, but I can see that happening to a lesser extent from a high rpm air dam.

[Steve]

Air is going in and out of the breather tube with the rise and fall of the underside of the pistons, and it is not completely canceled out by the configuration of an inline four.

Not sure about your inline four, but on all of mine, two pistons are going UP, and two pistons are going DOWN, the same amount, the same speed, so they will cancel each other out.

It allows the pressure to go out but meters a small return to maintain a 3 to 5 lb. vacuum in the crank case.

I have seen you post about this before, but still have not seen an answer to a question that was asked before: is there a pump in the system? How does it maintain that vacuum?

I have watched a Harley (early model Evolution) stop leaking oil to barely seeping as soon as the Hayden vent is put on the vent tube.

I can understand that in a V-twin, as the pistons go up and down somewhat together, separated by only 45 degrees of rotation. If there is a check valve in the system, the V-twin pistons will pressurize the crankcase on their downward stroke, forcing air out the check valve. When the pistons go up, the check valve would close, and yes, that could cause a vacuum. I just don't see that happening in an inline four.

Now, what is the advantage of having a vacuum in a V-twin? Since both pistons are trying to go up and vacuum is trying to hold them back, it will actually LOSE power.

.

[7981GS]

I am having a problem with oil leakage.
It was coming from the base gasket,
so I pulled the top end and redid the gaskets.
So no leaking from there, now its coming from the shifter shaft

[gearhead13]

HUH?
So, I went for a ride tonight and stopped for 5 minutes and had 3 little drops drip out
Vacuum in the crankcase helps in sealing the rings. Pressure from combustion forces the rings out to seal better. Any unwanted pressure in the crankcase would work against that and the rings would put less sealing pressure on the cylinder walls. A vacuum would compliment the combustion pressure from the top and help pull the rings down and out to help sealing.

[OldVet66]

Steve, You'll just have to try one yourself. you can feel the air going in and out of the vent. Stop the air from going back in, and the motor will pump it's own vacuum to a certain extent. The pistons rising and falling in sets do not cancel it all out, or you wouldn't feel a thing at the vent. It works, maybe it's magic, I don't care. If you ever get down this way I would be glad to let you try it out for yourself.

[old_skool]

Overthinking this, all pressure goes to area of least resistance, just replace the shaft seal, once you sealed up the first leak, another weak point let you know it needed attention as well, you don't need to re-engineer your bike, even with a larger bore, as Steve and Eric pointed out, your vent is plenty large enough. By the time you change the seal, your thread will be 10 pages long and someone will have you changing the back tire to a larger size if you look for other answers.

[gearhead13]

Shifter shaft seal is ordered and on its way. But there seems more to it than just a borderline seal. And there is, because after I opened up the breathing holes, instead of a puddle of oil I only got 3 little drops of oil.

[gearhead13]

I thought I had this fixed after installing a new shifter shaft seal, but no
Went for a ride, stopped for a rest and more oil under the left side sprocket cover. Not a huge deal, but it is getting on my rear tire too
The only other thing I can think of is the starter o-ring Dammit, summers going to be over before I fix this

[49er]

80mm is a big bore. What wall thickness do you have left on your sleeves?

If you're under 1.5mm, your rings wont be liking the experience, causing blow by when the engine is under high rpm load. You shouldn't need to open the breather up, but don't choke it up by fitting too fine a mesh gauze either.

If you're getting oil on your rear wheel, make sure that the base gasket isn't still leaking.