Oil tank blow-by

I am starting to get pissed with the oil blowing out of my oil tank, if I ride between 60-80 mph its nice and dry, any length of time around 90 and it starts blowing oil out of the filler cap and then down the right side of my bike, the oil level is kept about half way betwwen the marks. Can I pull the breather line off the oil tank and run a straight hose, or do they start pumping oil out the breather ;std 850, which would eventually empty the tank, my goal is to try and reduce tank pressure. second quick thought, PCV valve from oil tank to inlet manifold :?: :?: :idea: Any ideas or pics much appreciated and I know it wouldnt leak if i slowed down :roll: :roll:

Hi Splatt,

The oil tank shoud not be getting that preasurized. Did you check that the tank breather hose is not plugged. I believe you when you say it's coming from the filler cap, but also check to see if it isn't leaking at the chain oiler pipe.

I've run by '72 above 100 and never had this problem.

Hi,
I disconnected the breather line from the oil tank years ago and have had no problems with it. It just makes sense, the oil is contaminated soon enough from the combustion/blow-by process, no point in piping the rest of it directly into the oil tank. Besides reducing contamination I can fill purposely overfill the oil tank without worry of it being pressurized out the tank vent tube.
On the other hand I was not having any sort of pressure problem to begin with, if you are, perhaps you should check the vent as suggested and/or run a cylinder leak down test.

Splatt, What do you have running off the vent from the tank, that would be the dome at the top? Are you routing it to the standard aircleaner? You could pull the breather hose as you said and run it out the back of the bike like the old Triumphs, not too environmentally friendly. I don't mean the climate so much as the people driving behind you. Or you could hook it up to a vented catch bottle like a lot of race bikes. As far as running a PCV valve from the tank to the inlet manifold, the MkIII's do something just like that, but without the PCV. They used something that looks like a reverse hooka. Look up Group 20 on Old Britt's site for MkIII's and click the dinky picture at the top. It keeps coming back to the basic question though. Why are you getting that much pressure in the tank in the first place. That cap on the tank has a spring loaded gasket that would take some serious pushing to blow off the way you describe.

I'm experiencing the same mess coming from the oil tank. It seems to be coming from either the cap or one of the oil pipe connections. (Mk3 with a 34 Mikuni)

I've fixed the left side exhaust rocker cover leak.
Then it leaked from the right rocker cover, fixed that.
Then the Tacho drive, fixed that.
Then the screw plug in the timing case, fixed that.
Now the above mentioned oil tank. What's next?

I'm waiting on a PCV valve in the hope that will stop this game of leak and fix.

the standard Mk3 breather hose is connected to the oil tank, then the hose that used to go to the air box now runs to atmosphere with a small air cleaner.

If I fill to the full mark it spewes oil out the small air cleaner.

Any suggestions appreciated.

graeme.

Graeme, I think very few experienced owners fill their bikes to the full mark, Commandos seem happiest running at about the half way mark. I seem to remember there was also a report of some dip-sticks with inaccurate (too high) markings.

I was going to mention this earlier that it might be over filling.

Dave
69s

Thanks fellas, I had adjusted the oil level to half way between full and low and that stopped the oil spewing out the breather filter.
But not the other leaks.

Splatt, sorry for jumping in on your story. But it sounds the same as my slippery problems.
(although it does make it easy to spin the bike around on the centre stand)

graeme.

I'm pretty sure that when you fit the one way valve, salvation will be at hand.

Fullauto said:

I'm pretty sure that when you fit the one way valve, salvation will be at hand.

Click to expand...

Mine has been a blessing.

I hope this doesn't sound too silly, but I've seen this problem thrice before and each time the cause was simple. The oil tank cap wasn't tighted correctly

Just incase,
Cash

cash said:

I hope this doesn't sound too silly, but I've seen this problem thrice before and each time the cause was simple. The oil tank cap wasn't tighted correctly

Just incase,
Cash

Click to expand...

The cap doesn't seal very well, :shock: when I first got the bike the seat used to push on the cap. The tank breather runs out over the number plate, good for those speed cameras,there is no problems with engine oil leaks,yet, piston,rings, valve guides all new last year.
The top of the tank were it seals is very thin and I dont think this helps, but it doesnt leak at legal speeds, unless I dont put the cap on propely
I would like the breather to by pass the oil tank like triumphs but I feel it may pump a fair bit of oil out, can only but try I suppose, :?:
I know I'M not alone with this sort of leak.