Breather Tube - How much should be blowing?

[Guest blatham489]

(Checked the archives first)

How much air should be coming out of the breather tube? I never noticed before last week how much air comes out in a nearly steady stream (when riding in tall grass and saw the grass blowing, put my hand under it and was amazed at the amount of air coming out). I don't appear to have any smoke in the exhaust and the bike runs fine, just wanted to check if it is normal for this much air to come out.

Also saw some posts on routing this much air back to the air box, any further info on how well this works? Seems like you might as well get more air pressure into the airbox if you're already using the power to pump it out.

[Pablo-75rus]

Someday I've been amazed by power of air flow frow breather tube too. It was on my old '99 YZ400F. I've sold this bike and it still rides great

In "Motocross & Off-Road Perfomance Handbook" by Eric Gorr they even suggest to use filter for breather tube...

[FFRacing79]

Just imagine every time your piston is on the downstroke, It must displace all the air under it.

I did an experiment with a pail of water and a clear breather hose.(got the idea from Nigel, the team Husky mech.) At idle it blew a small stream of bubbles, and as the rpm picked up it actuall drew a few inches of water up the tube. I was trying to see if a one-way valve might be needed(HP). I run a filter on mine(as Honda essentially does with their routing into the airboot). Tdub

[Vibeguy]

A slight restriction on the breather line and you can see why the decomp plug in the heads are blowing out.

[FFRacing79]

A slight restriction on the breather line and you can see why the decomp plug in the heads are blowing out.

Exactly!

[WheelsUp]

A slight restriction on the breather line and you can see why the decomp plug in the heads are blowing out.

Yup... and that forward top clamp under the left radiator is pretty vulnerable until you cover it with a bashplate.

[dirt_pilot]

I was just doing an oil change and noticed my breather tube was pinched flat by the clamp right under the left radiator. If this is closed of or significantly restricted how will it effect the way it runs? My bike stalled a few times at the track and was hard to start last time out and I had never had a problem before. I also found some sludge like stuff in my oil filter that I had never seen before, and the air filter was clean and it had only been 2 or 3 rides since I changed the oil. Could this have been the prob or should I look deaper?

[WheelsUp]

A plugged breather will absolutely cause problems, including forcing oil past the rings, especially on the exhaust stroke where there's no compression.

It will also blow out the half-moon caps on the left side of the head... similar effect to blowing the decomp plug... and oily mess and screwed engine if you don't notice.

[dirt_pilot]

Could it cuase the stalling and/or the sludge like stuff in my oil filter?

[WheelsUp]

I would think that it might cause stalling, but I would suspect water (or coolant) contaminating the oil and causing the sludge... especially if you change the oil regularly.

[YZ250F_Rider]

Just imagine every time your piston is on the downstroke, It must displace all the air under it.

I did an experiment with a pail of water and a clear breather hose.(got the idea from Nigel, the team Husky mech.) At idle it blew a small stream of bubbles, and as the rpm picked up it actuall drew a few inches of water up the tube. I was trying to see if a one-way valve might be needed(HP). I run a filter on mine(as Honda essentially does with their routing into the airboot). Tdub

.

This is how I routed mine, it is in the clean air section of the airboot. It keeps water out and supplys clean air as the rpms increase. Not only that, but my oil has consistantly been cleaner when changing it since re-routing it.

[R_Little]

My breather was pinched for years by my ELine skid plate.

I kept changing countershaft seals until I freed the the tube and the oil leaks stopped....

DOOHHH......

[grayracer513]

Just imagine every time your piston is on the downstroke, It must displace all the air under it.

I did an experiment with a pail of water and a clear breather hose.(got the idea from Nigel, the team Husky mech.) At idle it blew a small stream of bubbles, and as the rpm picked up it actuall drew a few inches of water up the tube. I was trying to see if a one-way valve might be needed(HP). I run a filter on mine(as Honda essentially does with their routing into the airboot). Tdub

How are you able to account for the presence of a vacuum in the crankcase with the engine running? In an ideal world, breather airflow should be a net zero, but with combustion gases, it should be a net outflow.

[FFRacing79]

How are you able to account for the presence of a vacuum in the crankcase with the engine running? In an ideal world, breather airflow should be a net zero, but with combustion gases, it should be a net outflow.

As the piston descends, it displaces the volume beneath it. On the upstroke, it replaces the volume. That's the best I can explain it with my limited knowledge. Had the water been drawn up farther, I would have installed a one way valve but I did not feel it necessary.

[grayracer513]

I'm mystified. The trouble is that even at only 1500 rpm, the upstroke, as well as the downstroke of course, lasts only 20 milliseconds. Considering the volume of air present in the crankcase, including the transmission, and the compressibility of air it simply seems impossible that either event could move the total displaced volume either in or out through a 1/2" breather before the pressure is reversed by the following event.

What kind of engine was it, and was there any source of manifold or intake vacuum supplied to the crankcase?

[FFRacing79]

It doesn't, thats why it only draws about 2" of water up the tube. 60 plus HP YZF450

[YZ250F_Rider]

I bet if you tested with something lighter than the water, say oh well dust it would get pulled a lot farther up the tube than 2".

[grayracer513]

It doesn't, thats why it only draws about 2" of water up the tube. 60 plus HP YZF450

So are you seeing water raised, then dropped, then raised again, or raised and held to a specific level?

[FFRacing79]

It held steady level at about 4500 rpm

[FFRacing79]

I bet if you tested with something lighter than the water, say oh well dust it would get pulled a lot farther up the tube than 2".

Maybe that is why Honda(and me) run a filter.