how much exhaust or air should you feel out bypass

6 replies to this topic
  • milomilo

Posted May 08, 2008 - 05:46 AM


on a 450f how much air or exhaust should come out of the black hose on the left bottom of the bike? i didnt think any but theres alot

  • sparkyz

Posted May 08, 2008 - 06:24 AM


That sounds normal to me, it will basically blow 450 cc's of air out every time the piston goes down and suck 450cc's when the piston goes up. It's the crankcase breather hose. The air moved on the bottom side of the piston has to go somewhere, and this is where it goes.

  • grayracer513

Posted May 08, 2008 - 06:27 AM


At an idle, there will be quite a bit of air movement, and it will seem like less as the engine speed picks up.

A 450 is called that because the top of the piston displaces 450cc of air as it moves up from BDC to TDC. But the bottom of the piston displaces the same volume of air as it moves down from TDC to BDC, and that is most of what you feel at the end of the breather. Something less than 450cc of air will pulse in and out if the engine is turning slowly enough. At higher speeds, the piston reverses directions too often to move that much air before the pressure reverses.

Additionally, the rings, even healthy rings, leak. Not much, but some combustion gas inevitably ends up in the crankcase and must be vented off in order to prevent a pressure buildup that would push seals out. This is the real reason the breather has to be there.

What would be abnormal is for there to be a distinct puff of smoke from the breather each time the throttle was opened. Otherwise, the breather should breathe.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Mike Martel

Posted May 08, 2008 - 07:00 AM


My 06 450 also blows a lot of air out while idling. I noticed it blowing weeds on the ground one day. If i could just figure out how to make it blow ...

  • milomilo

Posted May 08, 2008 - 10:51 AM


ok thanks, i was not sure if any big problems or normal. i know some gets by the rings but just way more coming out of it than like on any car i owned.

  • grayracer513

Posted May 08, 2008 - 11:55 AM


... way more coming out of it than like on any car i owned.

Any car you ever owned would have had a piston moving up the bore while the opposing piston moved down the bore. That cancels the pulsing you get with singles, 360 degree twins, a single crankpin V-Twins

  • hillclimbguy

Posted May 08, 2008 - 04:45 PM


You will feel some pressure coming out of that hose. Its normal. That is a vent coming out of the Head cover.
You may want to reroute it if you do a lot of trail riding and water crossings.

Related Content


James Stewart back on a YZ450F by YamaLink

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies

Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • * * * * *
  • 1 review
  • Last review by Tommyk_55
  • On January 15, 2017
WR Camshaft Swap Info - last post by jamesm113

WR Camshaft Swap Info

  • 0 replies

1998 yz400f will not idle. by trolley

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  32 replies


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  30 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.