Diffusor?


4 replies to this topic
  • Windseeker

Posted March 17, 2008 - 09:17 PM

#1

I searched the Technical and Jetting forums in vain on this subj. May be I got the spelling wrong or smth.

The guys in the garage here go as far as having made for themselves custom alluminium St.John cross-looking like devices that they install between (actually inside) the carb and intake "for a proper fuel injection and burning".

Does it really make any sense performance wise?
Has anyone any experience with suchlike devices?
:confused:

PS: they call it "diffusor" ! :excuseme:

  • William1

Posted March 18, 2008 - 02:37 AM

#2

It is a gimmick to help resolve poor jetting. Same concept as a "Power Now" type of device. If a bike runs better with it, you can be sure the jetting is incorrect and the band-aid insert is confusing the carb into providing an improvment. Correct jetting will always work better.

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  • grayracer513

Posted March 18, 2008 - 07:48 AM

#3

The original Power Now, and other such devices were designed to address a particular problem with slide valve carburetors. At part throttle, from just off idle to around half, a turbulent roll over of air is created as the inflowing air runs up against the slide. The Power Now type device is a simple, single blade running across the carb intake bell horizontally, running down to just in front of the slide, and dividing the air intake into upper and lower sections. As the slide then opens, only the air in the lower half begins to move, and since the size of the divided air throat is much smaller, the air mass, and thus the volume of turbulence in front of the slide, is considerably less.

So what? In a relatively small, or high strung, aggressively cammed engine like a YZ250F, that has a built-in problem with low speed operability, this kind of device can and does often improve the performance and crispness of the engine at lower, part throttle openings. It does not add anything to peak power in any engine, and if not built correctly, can actually hurt peak power under some circumstances. Generally, in big torquey engines like YZ450's, it won't help. Even in later models with "softer" lower midrange power curves it will do nothing to improve the hit, since the softness is a result of ignition mapping (or the exhaust, on an '08), and not due to any problem with air flow at the carb mouth.

  • Windseeker

Posted March 19, 2008 - 05:21 AM

#4

Thank you guys. With your permission I will quote you at our next evening garage gathering. :confused:

The other bike they are installing it on is a freshly rebuilt Kawi 250 I think 2006, that already has a JD kit on and a good pipe... Should be good enough without it...

  • todds924

Posted March 19, 2008 - 06:16 PM

#5

Thank you guys. With your permission I will quote you at our next evening garage gathering. :confused:

The other bike they are installing it on is a freshly rebuilt Kawi 250 I think 2006, that already has a JD kit on and a good pipe... Should be good enough without it...


A guy i know with a WR 450 swears by the "POwer now".....Better low end he claims........I havent checked one out for myself.





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