Breather return, to ground or tank?

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Dec 19, 2004
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Greetings All,
I was reading the last couple of postings in the "Mobile One" thread, several spoke of different breather return routings. I used to run the stock set up on my 74 then later I installed the one-way brake booster valve (as discussed in a lenghty thread on the NOC site). When I did the installation I used clear vinyl tubing from the valve to the tank, after some time I was concerned with amount of condensation that was visable in the tube. It occurred to me (after a re-read of the NOC thread) that the engine pumps not only condensation into the oil tank, but, alot of engine blow-by gasses too. I have since re-routed from the valve down to the ground, and a separate tank vent hose also to the ground. The question is, does anyone else think that the condensation and blow-by gasses are worth worrying about, (I mean they still get into the oil) I thought perhaps by routing the breather to the ground it would at least reduce the amount?

That concerned me too and while I don't have clear pipe on mine it is fed from the one way valve via reducer to the filler neck stub. The tank then vents into a small plastic bottle next to the battery. This fills up at about 5-10cc every 100 miles depending on humidity and type of riding. Mostly the content is water with some "mayonnaise". The one way gets cleaned every now and then and is usually full of "mayo". Of course some is left in the tank and it looks pretty nasty when it exits the tank drain on an oil change. Not sure what we can do other than ride harder?
Based on my observation of the oil mist which blows into the oil tank from the breather from cold (noticeable when checking oil return with the cap off) I have always, perhaps rather conservatively, assumed that breathing into the oil tank was a good idea.

I tend to think that it just highlights the value of giving the oil tank a good clean out annually. I have no experience of synthetics in Commandos but I don't think that its ability to do high mileages without deterioration should be used as an excuse for not doing very regular oil changes.

That said, any water in the tank sits at the bottom well below the level of the oil outlet so perhaps it's not doing any harm ?
breather gsses

Hi Again,
I had to laugh the "Mayonnaise" description is quite fitting. If water vapor was the only ingredient I wouldn't worry about it (so much). Most modern automotive crankcase ventilation systems use manifold vaccum to more effectively remove those fumes. Oil in transmissions and differentials lasts many many miles because it is not being diluted by blow-by which forms various acids in the oil, I figuered no sense blowing it directly into the oil tank. On the other hand most bike people (especially classic brit bike people) probably change their oil frequently enough so that it is not a problem.
I have tried every breather arrangement going and ended up with a large diameter pipe (about 1/2 inch) straight from the breather stub on timing case to exit behind rear numberplate, no catch bottle, valve or anything. No oil mist builds up & engine is oil tight. I think dia of pipe and length allows oil mist etc to drop back. I had mayonaise when I tried valve & ditched that idea
breather ??

Hi Geoff,
Hhhmmmm...thats an interesting observation, I too had not noticed the "mayonnaise" until the addition of the oneway breather valve. Could it be that the theoretical gains supposedly had by installing the valve (IE reduced pumping losses and increased oil retention) come at the expense of an inadequately vented crankcase. Your breather tube to the back fender (ala Triumph) might not be a bad idea (unless any of you know any Triumph breather horror stories???)

I've been keeping to the sidelines on this discussion...but ....."the devil made me do it"...:twisted:

Isn't it a bit much to consider putting a tube out the back, like they did before anyone cared about whether half of the oil thrown out the back, ended up in the river? Don't these things leak enough? Why produce another one.

Thought the days when we dug a hole and dumped our old oil in it were dead and gone...with good reason. Since we decided to watch a bit what we do...we have fish you can catch and eat, again in our river, otters on the sunny banks, ducks and turtles in the water, you can canoe and the kids can "go down to the river" and skinny-dip again, like it all should be. I'd rather have a bit of mayo in the oil tank and pay for an extra oil change, than dump it out the back and know a litre of my mess will screw up a million litres of my and my grandchildren's drinking water...route the stuff where it should go, not out to the street. Reconsider. Do it right. That way you will have a clean conscience.

Greetings He Who,
Point taken, and, I asure you that I am not dumping oil. Perhaps we are all alittle guilty (those of us on this forum), after all, we hold on dearly to our 30+ year old technology, when we could be driving fuel injected, charcol canistered, catalyzed exhausted, air injected, low emission, completely oil tight, wisper quiet, modern motorcycles....or not driving at all.
yep we could be driving something more modern but think of the resources that it takes to build a new vehicle keep the olden's running all that waste if you scrap them and as far as mpg goes my 350 Velocette does 70 mpg and that's thrashing it.....can't thrash the commando in SE England not quite enough road :mrgreen:
Right...we are all guilty. We could all be driving one of the new spiritless machines....but that's not my point. There is something we can do about some of the imperfections of a thirty year old machine, and if we can, we should. If we expect to continue to be allowed by law and legislation the enjoyment of driving an old, loud and at the very least, antiquitated machine...for the pure fun of being fast, loud and different than all the others on the road...we can try to make it as drip-free and non-polluting as possible. To at least keep the machine up to the standards to which it was built. Adding a cat and such isn't what I mean. I mean keeping seals and gaskets in order. Nothing spoils the pleasure of seeing a rare car or bike, more than seeing a puddle of oil under it. Somehow it just ruins the effect. My bike has had as many leaks as the next one....but I have set myself a goal to try to elliminate them...nothing cool about oil under the bike. I might be flogging a dead horse here....but the thought of a Commando cooking some corner in the mountains, leaving behind a mist of sump oil sprayed from a tube mounted on the fender...just doesn't quite fit the picture...
Sorrry about long delay replying as not been on site for a while. Hew, point is that great long breather does not dump oil mist anywhere, it is pointing at rear of numberplate & there is not a trace of a greasy film! I think using more closed system & PCV does not allow water vapour to escape, & you see similar mayonaise in cr systems as well. If engine is reasonably well put togethre oil leaks are not a problem.
Another thought! Thats 2 tonight, I must go and have a lie down! IF we were right on totally PC genuinely caring about the planet types, we would get rid of any vehicles that were not absolutely necessary & certainly not jump on a fossil fuel burning device purely for the pleasure of the ride & without any point to the journey. How many of us have commando as sole means of transport & only use it for essential journeys? I do agree that old bikes are more fuel efficient than modern types, and to hear modern motorcylists getting on their high horse about how much more enviromentally friendly biking is than cars is to shoot themselves in the foot when some sports bikes are doing fuel mpg in the 20's

I have absolutely no experience with the motors that are vented at the end of the cam....I will soon, as the Dunstall I have started to fool with is one of this type. My experience is only with the 72 type, vented on the back of the motor. Mine is a wet sumper, been for as long as I can remember. I never drain before riding, never have, and see no sense in starting to do so now. Mine pumps up all the oil it had in the sump, and due to the fact the tube is vented into the tank, none of this goes on the ground, and I keep the oil level at a good level to avoid dumping down the breather tube into the air filter. I suppose my horror vision is of all that oil just pumping out that tube on the end of the cam, and onto the ground...I seem to remember having seen it happen, years ago by one of those tube to the back machines. Am I incorrect? Will this not happen in the case of a wet sumper? Hard to think it wouldn't...but I've no idea. If this isn't so, and the oil won't be just pumped ou the back....where else would it go, and how long would it take to get there...long enough to blow every seal in the motor? Please enlighten me, as I have plans to route the tube on the Dunstall up into the tank too...wasted effort, or do I have a valid point?
HeWho, I have no experience with cam breathers either, but do with your rear breather and the 850 style breather.

Yes, when my rear breather sumps it pumps through the breather and the return. My rear breather always seems to have oil in it.

My 850 sytle breathers are much more clean. They sit higher than the oil level in the crank case and dont' pump back through and don't produce as much oil in the mist as the rear breathers do.

I still run my 850 breather to a catch can, then the atmosphere. The byproduct is very small, but it is there.

Mine is an 850 from 1973 & it wet sumps over 3 to 4 weeks, to the point where strainer is uncovered. I keep an eye on level if bike not being ridden for say a week & make sure it gets a journey even just 5 miles to keep oil in tank, If it sits around in the winter, I leave it empty pending fresh oil. I have never had it pump oil out of breather due to wet sump, I think my breather works well because it is large diameter tubing, &~ is routed with minimum of tight turns, but there is a vertical run just in front of the tool box, from level with breather stub on engine to the frame loop. I think this allows any oil spitting out of the engine to fall back. If yours wet sumps sooner than 2 weeks, perhaps your oil pump needs refurb?
Geoff and Mike...

Never had an 850, but I seem to get the impression from listening to you all, that the breather is much higher up on the cases than on the 72, which is really quite at the bottom of the motor. Might be that the 850, since the breather is so much higher, never has the oil level get to the point where it gets shoved up the breather pipe, as it has to in the case of the 72 type. The 72 breather is so low that any amount of oil will be forced up the tube by the pistons pumping against the air in the sump. The air only travels one that I have a PCV on the breather, used to go in and out with the rise and fall of the pistons, noisy and caused leaks. Leaks reduced with PCV in place. But, back to point, if there is a cup of oil in the's coming up the breather on mine, maybe not on yours if the breather is so high up, where I think it is. Might be all that gets pushed up your tube, is the vapor and the oil sits down there in the sump till the pump puts it where it should be, in the oil tank. That's a long time to have it in the sump, but presumably it doesn't cause too many pressure related leaks, as the air in the sump in open to the atmosphere via the tube. Whole different situation than having the oil level above the entry level of the tube, s in the cse of the 72, if I understand it as I do. And now the question, what does the breather on the cam do...does that pump oil up the tube too, like mine, or is it half way dry like yours?
As to whether mine wet sumps to the point of having the oil tank go dry...don't know. Never looked. Sometimes it has gone months without being started, but I never checked. Even then, never remember it burping oil up the breather tube for too long though...maybe 20 seconds or so at the most. I listen for the spitting and burping, and then I know all the oil is up the tube and in the tank...but never had it where it didn't pump anything but vapor at startup, unless the bike had been run just a short time before, as in the case of re starting at the gas station...naturally she isn't wet sumped then.

The reason I got into this discussion in the first place was because I assumed the conversation involved running a tube from a cam breather to the back fender, like they were set up years ago....who knows...someone they throw oil out on a wet sumped engine, or not? Might be the 850 set up is acceptable to run out to the fender, and mine, or the cam breather, isn't. I just had pictures of a wet sumper pumping all the oil out the back and on the road, like mine would if I ran the tube back there instead of into the oil tank. So I stuck my nose into the thread.

Someone jump in here a clear up the question about the cam breather...if you said....I'm going to get into the nitty gritty with the Dunstall and it is a cam breather... :wink:

The breather on the end of the cam is a timed breather. Vapors must exit the crankcase via holes drilled in the camshaft near the center of the engine between the cylinders. and down the length of the camshaft. The end of the cam turns a ported disc which opens the pathway to the breather tube (I assume) when the pistons are falling and closes the pathway when the pistons are rising.

In theory this approximates the function of the brake booster valve. In practice, between the smallish holes in the cam and the convoluted path the vapors take, it doesn't seem to be sufficient to properly vent the crankcase at higher rpm. That is why most racers using the early cases add vents in the timing cover and sometimes the rocker galley.

I have never seen the need to vent the engine directly to the atmosphere, always returning the vapors to the tank and venting the tank to the atmosphere using a catch bottle or a small filter vent. Little or no oil seems to to come out this vent from the tank.

You are correct that a lot of oil comes out the '72-'73 rear breather when wet sumped. I would never vent these direct to atmosphere. Even to a catch can would fill it an require emptying and adding more oil to the dry sump tank. You might get by with the early ported cam breather or the later 850 timing chest breather, but it seems both would create a lot of oil mist and increase oil consumption (leaks).
Your point about limited venting through the this saying I should look into 850 style breathing on the Dunstall? I have no big qualms about doing a few mods on that one...she has had the head polished by that chopper builder anyway, so I figure I can't mess it up any more than he has already.... :wink:
Any modified early twin that is expected to see some high rpm would benefit from better crankcase venting.
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