fmf jet querry

8 replies to this topic
  • Anthony CRF450

Posted August 16, 2005 - 02:38 PM


hi guys
i got my fmf powercore 4 /powerbomb header and i installed the supplied fmf jet kit on my 05 wr450. it runs great but just woundering about jets. the fmf main jet size 150 was recomended for below 3000ft (my main alt) this seems strage since it replace the 165 stock main jet. the bike runs great but am unsure why such a smaller jet is required ie. from what i read should it have not needed a 165 or larger. the kit came with other jets 140/145/155/160. should i try larger jets and see what it rides like? or leave it
any adivice appreciated. :D
Regards Anthony

  • bg10459

Posted August 16, 2005 - 02:47 PM


Don't know your bike or anything about it, but I would think they gave you the wrong jets. When you open up the exhaust and/or intake, the bike breathes more air, which requires more fuel (larger jets).

  • Joe Reitman

Posted August 16, 2005 - 02:52 PM


I have been told that better flowing engine "pulls" the air/fuel mix through faster and more efficiently, allowing same or more fuel to be delivered through smaller jet.

  • Anthony CRF450

Posted August 16, 2005 - 03:05 PM


thats interesting 2 different points of view. jetting is a science getting it right is a matter or trial and error. if anyone out there has a similar setup let me know what your running (FMF PC4/ FMF Jets specific of 05 wr450f labeled as stage 2 power up). The new power delivery was incredible, sitting very close to the seat she lifted like a blue stallion, but still i am interested in the jetting differences so post if you have any ideas.
PS this site is great :D

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  • clark4131

Posted August 16, 2005 - 03:23 PM


I believe the FMF jet kit uses a different type of jetting system than the stock Keihin. A buddy of mine used the same thing on his '03 after I recommended he rejet after putting on his Powercore 4. :D He only sort of listened to me when I said to get a JD kit. He got a kit alright, but it was the FMF. :D After a few weeks of trial and error, he went out and bought a JD kit and all was right with world once again:banana: ...SC

  • Joe Reitman

Posted August 16, 2005 - 05:04 PM


Here is a useful link- you may have found already- it is very informative.

More 2 stroke oriented but still good, I think :D

  • bg10459

Posted August 17, 2005 - 06:08 AM


Ahh, yes. I believe I have heard that FMF uses a different numbering system for their jets. That would explain the differences. As far as "pulling more fuel", I'm not sure it's possible. Like I said, more air requires more fuel, i.e. larger jets. I could be wrong though :D , it wouldn't be the first time.


Posted August 17, 2005 - 08:40 AM


FMF KIT is made by dyno jet and distributed under the FMF name they use a diffrent scale then normal stock so a keihin 145 can be a dyno jet 165 or a dyno jet 135

  • Anthony CRF450

Posted August 17, 2005 - 02:05 PM


that helps- the numbering scale is just diffrerent to stock ones.


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