I moved the ugly vent hose.

25 replies to this topic
  • mycr500

Posted April 18, 2003 - 05:11 PM


In an earlier post, it was stated ....

HEAD'S UP on that head vent line: When you start your bike, the engine SUCK'S IN air via this line. When the engine is running, it blows out

If this is the case, then where does the air come from to blow out all the time?

Didn't these hoses usually go to the air box in 4 strokes?

  • MN_Kevin

Posted April 19, 2003 - 04:10 PM


If this is the case, then where does the air come from to blow out all the time?

The intake >> you have positive crankcase ventilation.

I have heard of guys putting PVC valves in this drain line.

  • yamaharichey

Posted April 19, 2003 - 05:46 PM


I just read somewhere, but I can't remember where, that they were running the tube into the air box, anyone else see that?

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  • mycr500

Posted April 19, 2003 - 05:54 PM


In a car, PCV (Positive Crankcase Ventilation) is realized by returning the hose from the crankcase to a source of relatively high vacuum, usually the intake manifold.

However, in the "old days", the breather was returned directly to the atmosphere, although the output was faced backwards, which provided some vacuum effect when the car was in motion.

Here is what I think happens with a single cylinder motorcycle, however. There are four reasons why a positive pressure (above atmosphere) might exist in the crankcase:
1) blowby
2) atomization of the lubricating oil
3) rise in temperature of the air inside the crankcase
4) movement of the piston.

I think the last one is of interest here. The crankcase volume would seem to decrease approximately equally to the displacement every time the cylinder comes down. Conversely, it increases with upward piston movement. Therefore, it would seem that the breather hose would alternately suck and blow with each stroke. It seems worrisome to me that this hose is returned to the air, since contaminents could be taken in. It may be that at higher rpms, the air simply cannot move fast enough up the hose to enter in any one stroke, although even then dust could cling and be progressively moved up. At low rpms, it seems that air could move all the way up the hose if the volume of the hose were less than the displacement. This may explain comments about water sucking up during starting as opposed to running.

In any event, it seems like the best idea to me is to return the hose to the clean side of the air box. It is not clear what happens if you were to use a check valve, although the possible vacuum effect is probably minor since most bikes don’t sit around idling much of the time.

Any opinions on this?

  • 426Woodsguy

Posted April 19, 2003 - 06:16 PM


I read the same thing on Offroad.com
I like the T-vent idea. If water sucked up, it shouldn't go past the T, should it?
Do carbs vent the same way?

  • MN_Kevin

Posted April 19, 2003 - 11:29 PM


Last year, I routed my vent hose up and behind the # plate/headlight. I used clear tygon. You can actually see a small column of oil that builds up in the hose from the normal venting action of the engine. Also, due to typical water producing action of engines (that thermodynamis thang of temperature and pressure and humidity) the oil can look milky :) - and no I do NOT have a coolant leak if you should ask.

This has prompted me into looking for another way to ensure this line drains efficiently.

I am opting to install a tee into this head vent. One line going down as a normal drain path with a PCV or another form of check valve, and the other end high up (most likely into the airbox) w/ a small (K&N) filter.


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