Crankcase Breather (2008)

Hello all. So I have an 850 1974 with a smoking left side so I've pulled the head etc and found the inlet valve seal off. Hooragh! I'm checking everything else while I have the engine in bits including the hone (Thanks Norbsa).

I took out the air filter(Black plastic air box with an L shaped flat filter) and it's collected a fair bit of oil. The pipe that comes from the oil tank I thought was the oil overflow but am I right in thinking this is actually the crankcase breather? If so why would this be leaking oil and is it something I should worry about?

Many thanks,

It likely is from overfilling the oil tank. The bike will wet-sump when standing, so before starting a bike that has been sitting more than a few days, check the oil tank visually. If you can see oil in the tank, start it and let it run for 5-10 minutes. Shut it off and check the oil level in the tank. Add oil if necessary. I never fill the tank above the half-way mark on the dip stick.

If the tank shows NO oil, drain the engine sump and either strain and refill into the oil tank or discard and add 2 quarts of fresh oil to the tank before starting.

From your description of the air box, you have a MkII A! Where are you located?

Ron L said:

If the tank shows NO oil,

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If the oil has drained down to the sump, then the standard oil tank on the later Commandos is always likely to contain some oil, as the oil feed outlet isn't positioned at the very bottom of the tank.

So I would say that if most of the gauze strainer can be seen above the level of the oil in the tank then air will probably have entered the feed system, so the sump ought to be drained and the oil tank refilled.

Watch out for this habit that the black box filters and the re-cycling system have of drawing in oil. This will be sucked into the inlet tract and burn on to the valve stems which can result in inlet valves sticking. This is not ideal because they will then try to kiss other moving parts :shock:

79x100 said:

Watch out for this habit that the black box filters and the re-cycling system have of drawing in oil. This will be sucked into the inlet tract and burn on to the valve stems which can result in inlet valves sticking. This is not ideal because they will then try to kiss other moving parts

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Youngbutaclassic,

And to add to what 79x100 said; many Commando owners choose to redirect the hose from oil tank breather into a small catch bottle instead.

Thank you all. I'll look into putting a little catch bottle on the end. Maybe I can attach near my battery? I assume there's a need for some basic type of filter to stop stuff being sucked into the oil system?

Yes, I do have a IIA and I'm in the UK

Engine Breathing

The perceived wisdom in the NOC Commando Service notes is that you should never fill the oil tank more than half way between the low and high marks on the dipstick. There is also a rumour that some dipsticks were the wrong length !!. I have a 920 MkIIA with a single Amal 36mm Mk2 carb thus the normal air filter no longer exists. I fitted a length of pipe from the crankcase breather extending to the rear of the bike but with a one-way valve near the crankcase end of it. The valve was from Norvil and it seems to work well (although when I received it I blew through it and could not make the valve open - I stripped it down and found it was stuck !! - a gentle poke with a wooden skewer fixed it - I'm glad I checked it otherwise the pressure in the crankcase could have reached high levels).
As long as I don't fill the oil tank much above the 'low' level all is OK but anything much higher results in incontinence (the bike - not me !!!).

It's always a good idea to totally drain your bike, change the filter, nip everything up, pour in the correct amount of oil, fire up the bie and let it settle in to a good return flow, shut it off and measure the level on your dipstick and mark it for future reference.

In this instance, we assume your bike isn't misbehaving and puking oil for whatever reason. Otherwise, drop your level slightly as required to ventilate without overflow, THEN mark your dipstick.

THAT is your correct level mark, period.

mwoo,
I may have missed something here.
Perhaps you should run the breather through the valve into the oil tank oil seperator and the oil tank vent to ground, or into a bottle. I would think if you aren't then your breather pipe will spew out oil.

Cash

Oil Tank Breather

Hi Cash - yes you're right - that's what I've done - my previous text was misleading. The pipe with the non-return valve goes to the breather connector on the oil tank. The oil tank then has a 'vent' feed to the rear of the bike.

79x100 said:

Watch out for this habit that the black box filters and the re-cycling system have of drawing in oil. This will be sucked into the inlet tract and burn on to the valve stems which can result in inlet valves sticking. This is not ideal because they will then try to kiss other moving parts :shock:

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Is this a thing? And how does it do it? And how do you stop the oil sucking?

On my MK2a with black box I keep the oil level half way between L and H, standard breather routing, no excess oil, foam filter stays moist with oil and inlet valves are fine.