Crankcase breather tube


40 replies to this topic
  • bigred455

Posted August 05, 2008 - 08:45 AM

#1

I understand the concept of the breather tube,and I cut it and rerouted it to the top of the cases(bracket for the carb tubes).I did this so it does not draw anthing in, just incase I were to take a fall in high sand or water.Also since the bike is moving forward no air would be pushed in because of the way the tube is routed. The pulses are pretty strong anyway pushing out I also know the pulses are short IN AND OUT,not really enough time to suck anything in. I know a few riders are running filters on the end of the tube in the airbox,I have a question on that.

Since the breather tube draws in and pushes out on the stroke of the piston,I KNOW WE ARE TALKING VERY SHORT PULSES. Having a filter on the end of the tube,wouldn't that cause interference or unbalance especially on the up stroke of the piston,drawing in?. Especially if the filter is very dirty,the outgoing pulse is normal,but the incoming pulse is weaker because of the filter. Also if the filter is clogged and you have the outgoing pulse,I would think you have a great chance of blowing the cam cover gasket or internal seals. Also with a filter at the end ,I believe you will have some condensation inside the vent tube perhaps making it to the cam area.

I feel pretty confident routing the tube a little higher like the cases and facing rearward with no filter.

  • Wiz636

Posted August 05, 2008 - 01:51 PM

#2

I have been running a filter on my '06 for quite awhile with no issues. As far as the filter clogging up, keeping it clean is just a part of the regular maintenance of the bike.

  • grayracer513

Posted August 05, 2008 - 01:59 PM

#3

You're right about all this up to a point.

The breather tube is there to vent the crankcase so as to do three things; allow for thermal expansion, allow combustion blow by to escape, and to relieve the pressure, both positive and negative that results from the pumping action of the single piston.

The concern over the engine ingesting something through this hose is valid, but only at very low engine speeds. The tube itself has a capacity of roughly 220cc. If you turn the engine at cranking speeds, the down stroke will push 449cc of air out, then the upstroke will draw 449cc in. The first 220cc in will be air that was in the tube from the previous down stroke, and the next 229 will be new stuff from outside. At cranking speeds, there is plenty of time for this to happen, but even at an idle of about 1500 rpm, these pressure events only last for 20 milliseconds. Because air has mass and inertia, and because it is compressible and expansible, 20 milliseconds is not enough time to move 220cc of air through a 12mm hose before the pressure reverses.

There is also a net outflow in the breather resulting from combustion gases entering the crankcase past the rings and from escaping oil vapors.

So you are fundamentally correct that there is not a great risk of having the engine draw up anything from the breather, but wrong about the imbalance question. If there is an restrictive filter on this hose, the up stroke will produce a momentary vacuum, and the down stroke a momentary high pressure condition. While it is not desirable, it is in balance. The reason a filter is recommended when rerouting to the air box is that there is a lot of very fine dust there, the hose is horizontal and down to the engine, rather than mostly up, and the hose is shorter than it was, meaning that the inward pulse need not be so long as to would have had to be to carry dust into the engine.

  • cowboyona426

Posted August 05, 2008 - 03:21 PM

#4

You're right about all this up to a point.

The breather tube is there to vent the crankcase so as to do three things; allow for thermal expansion, allow combustion blow by to escape, and to relieve the pressure, both positive and negative that results from the pumping action of the single piston.

The concern over the engine ingesting something through this hose is valid, but only at very low engine speeds. The tube itself has a capacity of roughly 220cc. If you turn the engine at cranking speeds, the down stroke will push 449cc of air out, then the upstroke will draw 449cc in. The first 220cc in will be air that was in the tube from the previous down stroke, and the next 229 will be new stuff from outside. At cranking speeds, there is plenty of time for this to happen, but even at an idle of about 1500 rpm, these pressure events only last for 20 milliseconds. Because air has mass and inertia, and because it is compressible and expansible, 20 milliseconds is not enough time to move 220cc of air through a 12mm hose before the pressure reverses.

There is also a net outflow in the breather resulting from combustion gases entering the crankcase past the rings and from escaping oil vapors.

So you are fundamentally correct that there is not a great risk of having the engine draw up anything from the breather, but wrong about the imbalance question. If there is an restrictive filter on this hose, the up stroke will produce a momentary vacuum, and the down stroke a momentary high pressure condition. While it is not desirable, it is in balance. The reason a filter is recommended when rerouting to the air box is that there is a lot of very fine dust there, the hose is horizontal and down to the engine, rather than mostly up, and the hose is shorter than it was, meaning that the inward pulse need not be so long as to would have had to be to carry dust into the engine.

This post is enough to convince me to leave the dayum breather tube alone... thanks for the thorough explanation grayracer.

  • grayracer513

Posted August 05, 2008 - 03:44 PM

#5

I don't want to leave the impression that the engine won't pull water up and in if you stall or attempt a restart while the end of the hose is submerged, because it is likely to do so. While it's running, there's not much chance of it, though. Still, if you are crossing streams and such quite a lot, rerouting it, or, as I would do, T it off with one tube down, and the other to the air box, is a good idea.

  • bigred455

Posted August 05, 2008 - 05:35 PM

#6

Thanks for the comeback,awesome answer. What about the filter causing condensation ? I am not running a filter and never will,I am just curious. With a open line there will be less chance of condensation,rather than a filter. I also believe that the vent hose should be cleaned every other oil change.

Think of this ,you have a little dirt just under the bottom of the tube. You start the motor,a piece of dirt gets sucked up the tube a half a inch. That piece of dirt is stuck to the inside of the tube waiting for the motor to be kicked over again and again each time getting closer to the cam inlet outlet,everytime the motor is kickstarted.

  • grayracer513

Posted August 05, 2008 - 07:35 PM

#7

The filter would be more apt to reduce condensation by creating a barrier to atmospheric moisture entering the tube as the engine cools.

Your theoretical upwardly migrating chunk of grit would be in defiance of the law of averages, since it would be just as likely to be blown down as pulled up during starting. It's possible, but extremely unlikely.

Since 2003, I have never found any trace of dirt or sand at the you of the breather tube, nor in the breathers of any of the multitude of thumpers I've owned and ridden in the past.

  • bigred455

Posted August 05, 2008 - 08:29 PM

#8

[quote name='grayracer513']The filter would be more apt to reduce condensation by creating a barrier to atmospheric moisture entering the tube as the engine cools.

Your theoretical upwardly migrating chunk of grit would be in defiance of the law of averages, since it would be just as likely to be blown down as pulled up during starting. It's possible, but extremely unlikely.

I totally agree FAT CHANCE,but there is a chance if the stars are lined up that day.

Now the condensation with the filter on, I would believe it would be more.The filter is no way letting the fumes and heat escape like the stock set up. Run the tube straight from the cam cover to the airbox you are even going to have more condensation,than if you had the tube pointing down,for good evaporation. Too each there own,good conversation:thumbsup:

  • cowboyona426

Posted August 05, 2008 - 09:28 PM

#9

I don't want to leave the impression that the engine won't pull water up and in if you stall or attempt a restart while the end of the hose is submerged, because it is likely to do so. While it's running, there's not much chance of it, though. Still, if you are crossing streams and such quite a lot, rerouting it, or, as I would do, T it off with one tube down, and the other to the air box, is a good idea.

Once you get some carb vents up into fresh air I've never had a problem with my bikes dying in the water. Not to say it can't happen, but it hasn't happened to me.

  • grayracer513

Posted August 05, 2008 - 10:01 PM

#10

Now the condensation with the filter on, I would believe it would be more.The filter is no way letting the fumes and heat escape like the stock set up. Run the tube straight from the cam cover to the airbox you are even going to have more condensation,than if you had the tube pointing down,for good evaporation.

Remember how condensation occurs; warmer air coming in contact with a cooler surface. It's the outside air that entered the engine as it cooled down on the previous heat cycle that was the source of the moisture. As the engine warms up, before the tube itself gets hot, that moisture produces the condensation. Once the engine reaches operating temperature, there is very little water vapor in the air within the crankcase, so the question of what can or cannot escape becomes less an issue than what can get in.

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  • Wiz636

Posted August 05, 2008 - 11:20 PM

#11

All the mathematics concerning the volume of air going up and down the breather tube at different speeds not withstanding...dirt will get all the way up the tube and drop into your head.

This pic is from my '06 at about 20 hours when I did the initial valve clearance inspection. The breather tube was in stock configuration. There had been no sand riding...just regular ol' dirt bikin'.

Maybe my '06 was an anomaly, maybe not. All I know is that I will not ride wthout a filter on the end of the breather tube again.

Posted Image

  • cowboyona426

Posted August 06, 2008 - 05:27 AM

#12

Stupid filter at work... can't see the photo. 20 hours (pathetic I know) on my 06 and no signs of "dirt creep" up the breather tube yet.

  • grayracer513

Posted August 06, 2008 - 05:46 AM

#13

Wiz,

Maybe you can help me out here. The only dirt I see in the picture is on a gasket surface, where it could not have gotten during operation (disassembly, yes). Point me to what I'm supposed to be seeing, please.

  • Wiz636

Posted August 06, 2008 - 08:07 AM

#14

In addition to the circled grit there was even more down below but my auto focus camera would not let me get a clear pic of it. It was directly below the spigot to which the breather tube connects. Whether it was a normal occurance with that particular bike or whether I dropped it while running in some loose dirt and forgot about it I couldn't say. All I know is that dirt was in my head directly below the tube connection. I did not like that.

Posted Image

So I put one of these on the end of the breather tube

Posted Image

and routed it like this...not totally sano for sure but it gets the job done.

Posted Image

  • grayracer513

Posted August 06, 2008 - 08:46 AM

#15

In addition to the circled grit there was even more down below but my auto focus camera would not let me get a clear pic of it. It was directly below the spigot to which the breather tube connects.

I see it now. Looks a little more incriminating. Was there dirt in the hose itself?

Even if the dirt got sucked up as you suggest, the circumstances for that to occur would have to be that the bike was stalled or restarted with the hose sitting down directly in loose dirt, or some dirt got stuffed up in there prior to starting. But just because it's not likely doesn't mean the potential for it isn't there.

  • Wiz636

Posted August 06, 2008 - 09:00 AM

#16

I see it now. Looks a little more incriminating. Was there dirt in the hose itself?

Even if the dirt got sucked up as you suggest, the circumstances for that to occur would have to be that the bike was stalled or restarted with the hose sitting down directly in loose dirt, or some dirt got stuffed up in there prior to starting. But just because it's not likely doesn't mean the potential for it isn't there.


Yes, there was some dirt in the hose. I don't recall any specific instance that would fit your above mentioned scenario, however, I crash far too often to remember them all so I can't say that it definitely never happened. Either way, I wanted to take steps to ensure that it did not happen again.

  • grayracer513

Posted August 06, 2008 - 10:16 AM

#17

With the climate around here, the only really common hazard is fine sand. Based on my experience, I feel no great urgency to modify mine, but it's on my to-do list for the summer.

I plan to leave the hose where it is and add a T to it, running the teed leg into the air box, probably with a filter such as yours. The breather will blow straight through the T, and the air box hose will connect at the center point. Then, if I can find one that suits me, I plan to add an in line check valve, possibly a PCV valve, to allow out flow in the lower leg of the hose, but prevent in flow. The valve can be something as simple as a rubber "reed" type, and probably isn't even necessary. By branching the line like this, any vacuum signal in either side is only half of that which is present in the hose at the can box, and any attempt to lift something heavy up the down tube will be shunted off by the air flowing in from the air box hose.

  • _JoNeS_

Posted August 06, 2008 - 10:52 AM

#18

Like gray says, I'm going to try the T method. Just for piece of mind, as I am planning to do some Cross Country which with out current climate may involve lots of water crossings etc.

Posted Image

I presume the setup would be similar to this diagram below ? I take it having the T peice this way up would be the most effcient ?

Posted Image

  • bigred455

Posted August 06, 2008 - 11:14 AM

#19

I am still not going to run a filter,like mentioned. I am running my vent tube through the engine mounts down on to the cases with the carb tubes.

I am cleaning mine out every other ride. I have a theory that each time you start the motor dirt inches it way up the tube,for how reliable the YZ valve trane is you would not think this is a problem.

  • bigred455

Posted August 06, 2008 - 11:45 AM

#20

Wiz I am not doubting you,but I have to ask this. Before you took off your cam cover did you wipe under the frame and surrounding areas?,Taking off the cam cover is a little tight and if it is not clean all around,it is really easy to knock dirt in. I never did have dirt around the cam towers etc,but I did find 2 specs of fine dirt at the top of the vent tube.





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