Swapping carb on 2004 wr450

6 replies to this topic
  • dan_zombie

Posted January 23, 2014 - 09:32 AM


Can anyone explain the advantage of the keihin fcr mx carb?

I have a 2004 wr450 with a very problematic carb. I don't want to shell out the ~$700 for a new fcr if I don't have to. I have tried to clean/rebuild and had it done professionally but I'm fed up.

How would a 39 keihin pwk carb work on a wr450? Or a similar option

If anyone has any actual knowledge on these you're help would be much appreciated!


  • grayracer513

Posted January 23, 2014 - 09:53 AM


You don't have to shell out $700.  They're reasonably available in very good condition on eBay for around $200, and since you have an '04, even the bare carb would work for you, as you already have all the right cables and hot start.  Look for one from an '05 or up YZ450.


With that said, the carb on the bike IS an FCR MX already, except that it has the AIr Cut Valve added to reduce backfiring caused by the air injection systems.  Without that setup, the pilot air circuit is simpler, and no modification to the ACV is needed (as there isn't one).  Makes the carb a little easier to tune.  If yours is in good condition, you can work with it if you choose to.


The PWK is built primarily for two-stroke applications, so it will have different needle jet tubes, air jetting, and would probably be a nuisance to get working exactly right.  It also lacks the rollers on the slide that help keep the slide working freely in spite of the very powerful intake vacuum the big singles produce on decel.

  • dan_zombie

Posted January 23, 2014 - 10:46 AM


Thanks for the response man.
The 700 was for a new one. I will have to do more research on the ais, but I'm 99% sure the po removed it.
Maybe you can take a look at this for an opinion. Last time I attempted to clean it myself and I'm pretty certain I messed something up on the accelerator pump linkage( don't know how ) ImageUploadedByThumper Talk1390502661.742988.jpg ImageUploadedByThumper Talk1390502704.691899.jpg ImageUploadedByThumper Talk1390502728.476464.jpg

Would I be better off to get a new used one or try to see if someone else can fix it? Sorry for the barrage of questions. I just wish I would have bought an efi bike!

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  • grayracer513

Posted January 24, 2014 - 08:02 AM


There are a couple of problems with that, alright (see the pic).  Download a manual if you don't have one, and look over page 4-15 and 4-16 for how that should be assembled.






What's supposed to happen is that the throttle will rotate the follower (bright metal link) clockwise on opening.  The timing screw then pushes the connecting spring, which pushes the rod link (black plastic part), which pushes the pushrod (missing) down.


The inner spring should return the follower counterclockwise against the shaft, and the connecting spring should hold the rod link up against the timing screw.  The pushrod must be in place as the rod link is installed so you can engage it as the link goes on.

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  • Krannie McKranface

Posted January 24, 2014 - 08:57 AM


Send it to Tokyo mods or AS Racing for a complete re-build AND jetting AND 2008 upgrades for $200-300.

  • dan_zombie

Posted January 28, 2014 - 06:34 AM


Thanks for the help! The follower on the wrong side was my problem. I have the timing screw& pushrod. Thanks for the suggestion krannie I'll give them a call! If I can get it fixed and jetted for the same $ I'll take that any day! And thoughts on what could have flipped that follower? Didn't disassemble that side at all !

  • grayracer513

Posted January 28, 2014 - 07:46 AM


Somebody did.  It will drop to that position by itself if the throttle shaft is removed.


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