Purpose of crankcase breather

Maintenance, repair, troubleshooting, etc.
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Crimson
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Thu Aug 10, 2017 5:24 am

Hello everyone

Last night I was thinking about engines, when I thought about the crankcase breather hose that goes from the right-side of the XJ600 engine to the air intake box. I thought, what would happen if I just disconnected this hose from the airbox side, and instead just tucked it away so rain wouldn't enter?

From what I understand, air-cooled engines have significantly wider clearances around the piston rings and cylinder wall. This is to tolerate a wider range of differentially heat-expanded and contracted components than a water-cooled engine. For this reason, they generally aren't manufactured to produce the same amount of power as water-cooled engines. Because the thermal expansion of components in them is less, water-cooled engines can run with tighter clearances, which permit a higher state of tune, which allows higher compression, etc.

Also, from what I understand, a crankcase breather system in any engine is designed primarily to allow blow-by gases (from between the piston rings and cylinder) to vent out. These gases contain no free oxygen molecules at all - they're mostly just noxious hydrocarbons. Now, because of the slightly larger clearances in air-cooled engines, there are even more blow-by gases to deal with than in engines with more effective cooling systems.

Given that blow-by gases are a fairly important consideration in the XJ600S/N engine, it makes little sense to me that these gases would be directed straight back up into the air intake box. That air leads nowhere but back into the engine. In effect, because it's not normal atmospheric oxygen-containing air, it must poison the air breathed by the engine. It would contribute to the operation of the venturi effect of the carbs, so the carbs continue to operate properly, but the air-and-fuel mix would be less combustible as it contains less oxygen, which would mean that as time goes on, with more and more blow-by gases by extending running at higher revs, the mixture would get richer. And probably have detrimental ancillary effects on the cylinder head, carbon deposits, etc. (in much the same way as an EGR valve on a modern engine). It probably also raises induction temperature after running for a while like that.

For these reasons, I was thinking of disconnecting it and letting it vent directly to the outside air.

What do you think? Is this oversimplistic?
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Crimson
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Fri Aug 11, 2017 6:36 am

OK, just in case anyone's interested, I got some rather good and detailed clarification on this issue from an engineer (Matt from The Workshop).

First, there's an error in my post as it mentions compression - the pressure exerted within the cylinder head changes but not the compression ratio.

Second, there is no appreciable effect on mixture by having the crankcase breather pumping hydrocarbons into the air filter, as carburettors metre fuel very roughly, and it wouldn't really make a difference.

Third, there is likely to be a coking effect on the piston and the valves from the oil mist pumped into the air filter, as it's an oil mist that pumps out from the crankcase breather hose. This would make a real difference over a period of time, as the air-fuel mix is dirty.

Fourth, the induction temperature effect would be negligible, especially because the ambient outside air temperature has a much greater effect, i.e. the bike runs differently in summer than in winter, because the air in the airbox is the same as the air outside, etc.

https://www.youtube.com/watch?v=zjH5hNAbQnM
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mikee112
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Fri Aug 11, 2017 12:51 pm

Crimson this subject has been covered before on the forum ( a few years ago now ), like you said the vent to airbox is not on the US Seca 2.... I found an article is Classic Motorcycle Mechanics Magazine where an official Yamaha source said it's OK to just plug the tube ( some folks had problems with it spitting oil into the airbox and clogging the filter )

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Crimson
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Fri Aug 11, 2017 2:19 pm

Thanks for the response mikee, but I think we might be talking at cross-purposes about 2 different things.

The crankcase breather hose is present on the USA model ("Seca 2") and appears to go up to the airbox. Just going by all the pictures I could find online, it's definitely there on the US model.

It can be seen on radare's scrambler:
http://xjrider.com/chevereto/images/201 ... _done4.jpg
And the stock image of a standard, all-original from motorcyclespecs:
http://www.motorcyclespecs.co.za/Galler ... 2%2001.jpg
Another shot of radare's bike show that it definitely goes from the crankcase straight into the airbox, on the US model:
gallery/albums/userpics/10002/headgasket10.jpg

I think you may be talking about something else: the weird drain hose that goes from the base of the air box, where dirty oil might collect, to a hollowed out bolt on top of the valve cover. I don't believe the US models have this feature.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

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mikee112
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Sat Aug 12, 2017 1:27 pm

Crimson wrote:
Fri Aug 11, 2017 2:19 pm
Thanks for the response mikee, but I think we might be talking at cross-purposes about 2 different things.

The crankcase breather hose is present on the USA model ("Seca 2") and appears to go up to the airbox. Just going by all the pictures I could find online, it's definitely there on the US model.

It can be seen on radare's scrambler:
http://xjrider.com/chevereto/images/201 ... _done4.jpg
And the stock image of a standard, all-original from motorcyclespecs:
http://www.motorcyclespecs.co.za/Galler ... 2%2001.jpg
Another shot of radare's bike show that it definitely goes from the crankcase straight into the airbox, on the US model:
gallery/albums/userpics/10002/headgasket10.jpg

I think you may be talking about something else: the weird drain hose that goes from the base of the air box, where dirty oil might collect, to a hollowed out bolt on top of the valve cover. I don't believe the US models have this feature.
Oh yeah... My bad, I thought you was talking about the valve cover to airbox hose :???:

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Unit562
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Sat Aug 12, 2017 11:09 pm

I actually prefer to either run a oil catch can system or just put those little Specter air filters on the hardlines. You quite often see very minute power shifts at power band. My civic gained 200 RPM on the top end before power dropped off after I installed a catch can system.
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radare
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Sun Aug 13, 2017 8:59 am

I've waited to weigh in. Here's my opinion for what its worth. This 'mod' does nothing for the engine. In fast, it modifies a very good engine emissions control that has been in place on cars for nearly 45 years. Removing it and venting to atmosphere dumps a lot of nasty combustion gassed to atmosphere, which, I like to breathe clean air. If you have an oil blowby problem, fix the problem. Don't band-aid it by removing the crankcase tube. :twocents:

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Crimson
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Sun Aug 13, 2017 2:26 pm

I don't have an oil blowby problem. I'm a maintenance fanatic, and there's no chance I would ride the bike if it had any problem of that nature. It would be opened up, and the parts on order. In fact, on my bike, you can't feel any oil around the breather hose after a long ride. But there is always some blowby with any engine. It's designed to be that way.
My POV is: it's only for emissions control, and it only dirties the air-fuel mixture, leading to increased carbon deposits in the long term. So it's fine to dispose of this pipe. Therefore I've decided to proceed with it. I've also decided to pop a special oil-catching filter over the vent, so that oil particles are caught and the atmosphere is kept clean.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

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