Jump to content

FCR-40MX Carb - Awesome Upgrade

Recommended Posts

I assumed that this might be the case.. 85 is actually moving pretty good for this bike. A slightly smaller rear sprocket may be in order. Or split the cases and get those second and fifth gear mods in..

Thanks dot.

Share this post


Link to post
Share on other sites

Ok, after combing through all 21 pages of this thread I didn't see this question addressed:

 

What are people doing with the vacuum lines from this carb? Are they necessary or can I just remove them all together?

Share this post


Link to post
Share on other sites

Vacuum lines or vent lines? there is a very important difference. my FCR "slant" doesn't have Vacuum lines and only two vent lines, I made a homemade filter that I run mine into because there was a considerable amount of dust getting into the top of my slide-which is bad...

Share this post


Link to post
Share on other sites

The half dozen hoses are that hang off FCR is what I'm talking about. Didn't know if people were routing them up or down the frame, to the airbox or not, etc.

Share this post


Link to post
Share on other sites

I can't remember if it was this thread or another one on ttalk. Four of the hoses are vent hoses and the one on the bottom of the float bowl is a over flow for the float bowl. I read to route two vent hoses one on each side of the carb up to the air box in case of a water crossing. The other two and float bowl overflow down between the front of the swing arm and back of the engine cases.

Share this post


Link to post
Share on other sites

I think he was pointing out that they're technically vent hoses, not vacuum hoses.  Except for the one from the bottom of the fuel bowl that's an overflow drain.  They're there for a reason, so don't start blocking them off.

 

It's probably best practice to route the two top loop hoses to the air box, but I've done lots of deep and long water crossings without the engine stalling.

Share this post


Link to post
Share on other sites

Normally all the vents lines are routed behind the back of the motor between the front of the swingarm.  That's how the OEMs do it too.  I've crossed rivers that completely covered the carb and never had any problems but rerouting them to the airbox makes some sense.  The only concern I can think of is routing it to the airbox would be the pressure swings might slightly affect the float operation and/or affect jetting. 

Edited by D0T-C0M

Share this post


Link to post
Share on other sites

Looking to make the switch also, found a good 07 250x CARB for sale, is there any benefit for a 250r carb over the 250x?

Share this post


Link to post
Share on other sites

Has anyone tested this setup at high altitude (8000 to 12000ft).

What changes if any did you make to ride this setup at those kinds of altitudes?

Share this post


Link to post
Share on other sites

WOW ramkas 8k-12k feet of elevation is very thin air.  For 10,000' elevation use,

 

NCVT or NCVS needle with the clip at the top most groove (1st from the top)

38 pilot jet

148 main jet

 

This should bring you very close.

Edited by D0T-C0M

Share this post


Link to post
Share on other sites

Dot

Thanks. I am riding out of Taylor Park, CO in July. Will use that as my base line.

Share this post


Link to post
Share on other sites

I picked up an FCR carb recently and im not sure if its a 40mm or 41. I measured the inside diameter on the inlet side on the carb and it measured 41mm. Does the throat taper down? Which way should I measure it? The seller said it came from a 2007 CRF250R which tells me its a 40mm. The main air jet is non adjustable if that matters.

Here are some pics.

image.jpg

image.jpg

Edited by craftsman13

Share this post


Link to post
Share on other sites

The carb will work that's all that matters..I always measure the inside diameter of the inlet part of the carb as to size..you seem to be doing it correctly..What do you mean by the air jets non adjustable???..That carb should have a hole on the bottom front edge around the bowl area.,inside the hole is the fuel adjuster screw. It should be able to be turned with a screwdriver of the right size. It's easier to replace the stock one with an aftermarket adjuster which pokes out the hole allowing you to adjust it with your fingers..

Edited by Horri

Share this post


Link to post
Share on other sites

y..What do you mean by the air jets non adjustable???....

 

the FCR-MX carbs has a fixed Air jet that needs to be swapped out for adjustment, the FCR "Slant" carbs have an adjustable Air jet.

Share this post


Link to post
Share on other sites

the FCR-MX carbs has a fixed Air jet that needs to be swapped out for adjustment, the FCR "Slant" carbs have an adjustable Air jet.

ok..got it..Any idea where it goes..Is their a hole or something that's plugged up for the slightly nutty US state emmision standards.

Share this post


Link to post
Share on other sites

Actually to be more specific there is a Slow Air Jet and a Main Air Jet, the slow air jet is adjustable on the FCR slant, I have read that the Fixed Slow Air Jet on a FCR-MX could be swapped out with a adjustable one.

I'm not actually sure what your asking "any idea where it goes?" unless you are asking what does it do? the Air jets are just part of the circuit for metering fuel for the pilot circuit  and main circuit, so there is no hole that would be plugged for emissions assuming that you would want as much adjust-ability to get the least emissions or power depending on your application.

If your asking "where is it on the carb" ? the red arrow points to the Slow air jet, the hole next to it is the Main Air jet.(you have to remove the bell in order to take them out)

 

ee083837-2831-47e2-86db-ce161e5e9123.jpg

 

100_2502.jpg

Edited by jjast

Share this post


Link to post
Share on other sites

Ok..Thanks for the detail..I was thinking they were like the fuel mixture screw that's on the front bottom of the normal FCR carbs..Bit of confusion but I think I grasp what you're saying.

Share this post


Link to post
Share on other sites

Thanks for the replies. There is no main jet on the carb I have, i hope tuning wont be difficult. I also picked up a NCVT needle and am waiting for some jets and seals in the mail. I think for a starting point i am going to go with;

NCVT needle w/ clip on the 3rd groove from the bottom

158 main jet

42 slow jet

68 starter jet

100 slow air jet

70 leak jet

These are the jets that came in the carb except the main jet so we'll see how it works out. If anyone has any suggestions feel free to throw them out there.

Edited by craftsman13

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:


×