Yz426f Breather Hose Air

3 replies to this topic
  • 5point9

Posted March 19, 2014 - 02:28 AM


So I bought this bike last summer and haven't ridden it a ton. But I went out to start it up for spring yesterday. It started easily (like it always has) and idled and seemed to ride out "normal" or very well. Very powerful and crisp, no smoke, very smooth.


This time, I noticed the amount of air that is coming out of the head breather! No oil, just a lot of air.. it's almost like a mini exhaust! At first I'm thinking that the exhaust valve has issues, but wouldn't compression suffer? Or performance? The bike runs well.. Is this normal? It makes perfect sense that the crank would make/suck air as the piston goes up and down, but man that's a lot of air!! It's like a small fan!


Also, I did some research and think this is "normal". If the bike is still, on the stand, stopped, etc... and I just whack the throttle open quick, fully, and hard it will just cut out and die. You don't really notice this when you are riding it as you always have some sort of "rev" going. Also, if I rev it slightly slower, she'll rev like a beast! It does have a 450 carb on it, and other than what I just complained about it carbs great.


Is that somewhat normal too??



  • grayracer513

Posted March 19, 2014 - 06:22 AM


Both normal.  The bottom of the piston shoves 426cc of air around just like the top does, so at low speeds, air blows from the breather as the piston goes down.  Put your finger lightly on the end of the tube, and you'll find that it draws air back in as the piston rises in the bore, too.  At higher speeds, the effect is less obvious, since there's less time before the pressure reverses.  There is usually a net outflow of air and combustion gas, as there is always some leakage at the rings, particularly in older engines.


Going from idle to WOT instantly in neutral will make a properly tuned 426 gag.  That simple.  It is possible to make it respond to such a goonish maneuver, but as you say, you don't ride that way anyway, and the problem doesn't show up underway if the bike runs right, so why?  Add to that the fact that it's more difficult to get the engine to run cleanly at low speeds/light throttle with a carb set up for that big an accelerator pump squirt, and it doesn't make too much sense.

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  • 5point9

Posted March 19, 2014 - 02:00 PM


Why? I just want to make sure there is not a problem with the accel pump or the like.


No, I don't just sit there and rev it in neutral. 


Thanks for the info.. always re-assuring to know the bike is acting "normal" especially since it had an unknown past.

  • grayracer513

Posted March 19, 2014 - 02:56 PM


The "why" was why would you try to set it up to be capable of idle to WOT maneuvers, not why do you want to know.

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