Complete Guide To Installing FCR MX on a XR600

Ok fella's

After doing a lot of research and not finding all the answers in one place, I have compiled everything you need to know for fitting the FCR MX to an XR600. I have all the Part numbers, prices, and pics of the finished install. She runs like a rapped ape now, Hands down best mod I have done to the bike since the rebuild last year. With a little tug on the bars it will wheelie in 3rd with 14-45 gearing. Second gear wheelies come up a little too fast for comfort. Ok on too the Technical stuff.

Step 1. Buy a good used FCR MX carb off of Ebay.

I got a really clean one from a 2007 CRF250 for $130 shipped

Try and find one with the throttle and cables, your XR600 throttle house and cables will not work.

Step 2.

You need a intake/insulator boot from a XR650L. Can be had on Ebay for about $30 shipped

This angles the carb in and down so it clears the frame and the tank. Without it the carb will hit the frame and the tank.


XR600 left XR650L right

Step 3.

Making the connection from the back of the carb to the Airbox. There is a about a 5/16" offset left to right from the carb and your airbox. You can connect this junction several ways. I initially used some plumbing supplies from the hardware store. About $4 in parts.

Basically a 1-1/2" PVC coupler and 38mm flexible PVC rubber joiner. Cut to fit and hose clamp it all down. Not the best but the cheapest and most straight Forward. I don't think the NOSS adapter that is sold will work very well here because of the Side to side offset created when using the XR650L boot. The Noss adapter seems to account for a vertical offset.





Now for the fancy part. Our engineering group just bought a 3D printer so I made my own custom perfect fit adapter. It is solid plastic. I can't say for sure how well it is going to hold up but initial test are promising. Gasoline/ carb cleaner seem to have no affect on the plastic. It is winter here so I haven't got any real world field testing done. There are several differenet types of plastics that can be printed so I might want to try a few different prototypes. Anyone who wants the file, pm me and I will send it to you. If you know someone who has a 3D printer have them make you the part. We have the "Makerbot 2" it took 6-1/2 hours of printing time to make it solid.












You will need to file off the lip of the carb in order to install the printed adapter. I used RTV sealant to glue it on. It's a snug fit, even without the RTV it's hard to get off the carb.

I used DOT-COM's Jetting suggestions and it runs great. Thanks for all your hard work buddy!

158 main

45 pilot

NCVT needle 3rd clip from the top

leak jet is still stock - I think it is someting like #65

Here is the total parts list and pricing.

2007 CRF250 FCR MX --- $130 Shipped - Ebay

2007 CRF250 Tusk throttle cables $14 -- Rockymountainatv FYI --- 4" longer than stock XR650L cables but will fit the XR650L housing. The longer cables fit much better. No binding or Stretching when turning.

Hot Start Button OEM parts 2007 RMZ450----Rockymountainatv 13410-35G10 STARTER VALVE | MODEL K5 $18.88


XR650L intake boot -- $23 shipped --- Ebay

Jets Prox brand from -- Rockymountainatv -- $15

NCVT Needle OEM parts 2004 YZ450F --- Rockymountainatv TA-14916-VT-00 NEEDLE (#NCVT) | $9.37

Custom printed part Freebie!!!!

Total Cost $210 give or take a little

If you have any question let me know. I will do my best to answer them. Enjoy the new found power.

This post has been promoted to a wiki

Edited by jtf1307tie

Looks great. The blue adapter is a nice touch. Looks like a race bike part.

Jets Prox brand from -- Rockymountainatv -- $15

NVCT Needle OEM parts 2004 YZ450F --- Rockymountainatv TA-14916-VT-00 NEEDLE (#NCVT) | $9.37

just to clear things up a little,one place says ncvt needle anorher says nvct .

I think ncvt is the right one.?

Very nice job man , !! love that adaptor, did you make any extras ??

Edited by winginit

Nice write up. I have a 07 250 fcr mx with throttle and cables on it's way to me. Also an L intake boot.

Should you print more of your adapters, I would gladly order!

Do the pictures show that you did a tiny bit of grinding on the carb on the upper rear section of the throttle "wheel" (the part where the cables attach)? On mine the metal body of the carb itself clears by a hair, but the plastic wheel cover just barely rubs in one tiny spot and needed to be trimmed. This is with a 650L manifold insulator boot. I'm interested to confirm that someone else is finding the same dimensions.

Edited by heart_of_darkness

Yeah a little bit

Can you tell me if thats a 37mm , 39mm, or a 40mm carb. and Thanks for a Great job on this , also if you make more of these adapters i'll buy , i think you could market these .

I didn't measure it, but based on the info I found that year crf250 should be 40mm.

Thank you for replying , You did a Great job and did all the foot work for me , I have been looking for a better carb. set up than the old stock one . I am now on the hunt . Again Thanks for the Great job.

Just an FYI for everyone. Early crf250r carbs are 37mm. 2006 to 2009 are 40mm for the 250r and all years for the 250x are 37mm.

Are the 37mm carbs the same exterior dimensions as the 40mm? The smaller size might be a better complement for low RPM riding.

JTF1307, how is your printed adaptor holding up?

Outer dimensions I believe are the same. My bike has more shit and git than I thought possible. Low rpm grunt is amazing. I have 400 off road miles on so far and things are holding up great. Doesn't seem to get warm, and that was what I was worried about.

After 2005, carbs for the 250r and the 250x are different beasts. Although the 2004 and 2005 X and R bikes where similar, hence why the X got great reviews and really kicked up thw trail bike market, after 2006 the bike started to seperate and the R became much more race focused and the X stayed the X.

So, the air boot, carb, and insulator are different on the later model Rs from the X. Basically, the R stuff is bigger than the X.

This sounds like an amazing mod for the XR and i can see a ton of us trying it this winter (watch out ebay bidders, i use super sniper), but you have to make sure you are buying the right stuff. You can make the 250x carb work, but i am not aure if the 650 insulator will fit. It might just need a little more crimping down on the hose clamp. Like i said, i am sure either can be adapted, but the later model 250R carbs are what you are looking for to fit this application.

On another note. Its no secret in the CRF world that the 2006 carb had issues. It never really shook the bog issues that the 2004 and 2005 had (although it can be fixed with an accelerator pump mod using an o-ring). If you can get a 2007 or later it would save some work for you. Or, get an 06 carb and do the AP mod for it using the o-ring or thin wire.

Just two cents to help folks not make a purchasing mistake. The only reason i know is because i tried putting 250R carb parts on my 250X and it was a no-go.

By the way, that XR looks flippin' awesome. Clean, but mean. Give you my Diet Mt. Dew for a test ride;)

I'll double his offer!

Edited by bork

As an alternative to the printed plastic adaptor, I've found that it's possible to do an XR600 FCR swap with XR650L parts: 650L insulator (as above), FCR carb, Noss adaptor, and 650L intake boot. The 650L intake boot is a direct swap onto the 600R air box, or the complete boot and airbox can be swapped (the plastic air box seems to be identical between the two). The 650L air boot corrects the lateral offset of the 650L insulator and is larger in diameter where it clamps around the carb adaptor.

The Noss adaptor is not a perfect fit. It's slightly too large in diameter where the air boot clamps, and is about half an inch too long which puts a slight kink in the air boot bend. I don't know if the 600R's frame geometry is slightly different from the 650L's, creating a worse fit, but those problems seem to be common to 650L carb swaps too. It's not too big a chore to trim down the Noss adaptor on a lathe.

Your above adaptor is a more elegant solution. Maybe the Noss company could use your dimensions to whittle them out of aluminum.

JTF, what gas tank are you using? Do you have any clearance issues?

Edited by heart_of_darkness

Great write-up... so good that you convinced me to take the plunge and snagged most of what I need to do the same mod on my xr!

What are your normal riding elevations? Temps? Relative humidity? (I typically ride in the coastal mountains of Oregon, so I'm curious about how the jetting factors compare between your locale and mine)



I have the ims tank. I got it used from an old bike I bought for parts. It sat high the front, the rubber frame pad never touched so I added some more rubber in the front and the back. This gave me all the clearance I needed for the fcr.

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