Alot of air coming out of breather hose

3 replies to this topic

Posted July 27, 2006 - 05:38 PM


I went for a little test ride in my running shoes tonite(I know it wasn't the best footwear) on my YZ450F. And realized that there was a fair bit of air comin out of the cylinder head breather hose. I haven't really rode noticed this before but i haven't really ever rode in runnning shoes. Anyways i was wondering if you could tell me if this is normal or not. Thanks

  • 2012 450 xc-w

Posted July 27, 2006 - 06:05 PM


Completely normal. Don't be fooled however, if you stall in standing water you could suck water into your top end. I recommend reroute mod. Do a search. Only difference is I did mine on the left side of the shock as I felt it was a better route.

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  • Zique

Posted July 28, 2006 - 11:29 AM


I ride in sneakers all the time. I don't like the feel of boots. I do wear some hardshell ankle braces though as well to offer me some protection.

  • grayracer513

Posted July 28, 2006 - 12:42 PM


450 cc's of air heads for the open end of that hose every time the piston runs down the bore. It usually doesn't have time to pass that much through it, though, because as soon as the piston goes back up, it wants 450cc's of air to fill the space underneath it. Added to this yin/yang thing are the small (hopefully) amounts of combustion gases that leak past the rings, which makes it a net outflow of air.

The danger of slurping water is both real and exaggerated. (?) The hose will not hold 450cc of air, or anything else, so if the engine were case deep in water and shut off, one good upstroke while starting could very well pull water into the engine. But there are three things working against this happening while the engine is running. First, water is heavy and hard to lift. Second, air will expand in response to a vacuum. And last, a running engine gives the water too little time to be drawn into the engine before the pulse reverses. When the piston goes up and pulls a vacuum on the submerged hose, the air "stretches" as if you were picking up a water jug with a bungee cord. Before the vacuum rises far enough to lift the water very far, the piston starts down.

So, it's almost always safe to cross water with the breather configured as is, but keep the revs up, and don't stall it.

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