Re-Carburating my 99´ YZ400

Hey guys!


As many of you I have Idle Problems with my old FCR carburetor in my 99´ YZ400. I tried everything, new hoses, all new rubberseals, cleaned the carb about 5 times and rejettet. Nothing worked.

Im planing on swapping to a YZ450F Carburetor, but as I´m from germany, the YZ450 carburetors are extremely rare and expensive. 

Can I use a FCR39 Carb from a KTM for example? Of course modelyear 2005 and earlier.

I know I will have to rejet the carb and refit all the necessary throttle and hot start cables.

Attached you will find a picture of a FCR39 Carb I could buy. It is a 2010 FCR39 Carb.


Thanks guys,


Carb 1.JPG




Edited by Domnaek

Hey, Check out the wear inside the carb where the slides rollers go.  they can wear and the slide will rattle at idle causing issues.  if its too worn out dump it for a new one

Should be usable, yes.  Just duplicate the OEM YZ450 jetting and go forth. 


But, be careful that the main air jet and pilot air jets are not overlooked in the process, and that the main nozzles are of the same size, or you may end up with a considerable calibration headache.

Alright, thanks a bunch!


I will keep you guys updated as soon as I get the carb.


See you then ;)



Hey there!


I am just one step away from finishing a deal for a 2007 FCR 39 carb. Unfortunately my bike is 500km away from me, so I can´t measure the outer diameter of the original intake manifold of the original carb.


Could yuu guys tell me the outer diameter of both the intake manifold from the engine- and airfilter side??


Thanks alot:)



That carb will plug in to your stock air boot and carb mount sleeve, if that's what you wanted to find out.  The later carb is a little shorter, so the trick is to clamp the air boot onto it first, then push it forward into the isolator sleeve, stretching the boot a little in the process.  



yep that´s exactly what I wanted to know. Just thought there might be some diameter differences of the manifolds within the fcr39´s. 

Anyway, thank you!

Carb is bought! Hopefully next week it will be shipped and I can check all the jets and order the right ones if necessary.


See you next week :D

Your baseline should be stock jetting for the YZ450. 

Is this the stock 450 Jetting?


Main: 160
   Pilot: 45
   Needle: NFLR
   Clip position: Third from top
   Fuel screw: 2-3/8 turns
   Leak jet: 55

Is this the stock 450 Jetting?


Main: 160

   Pilot: 45

   Needle: NFLR

   Clip position: Third from top

   Fuel screw: 2-3/8 turns

   Leak jet: 55

Not quite, but close.


OEM 2007 YZ450F(W) Jetting:

Main Jet: 160

Pilot Jet: 45

Starter Jet: 72

Leak Jet : 55

Fuel screw/Setting: 1.25-1.75 turns out

Needle/Position: NFLR-4 



I´m cleaning my new FCR carb and have just ran into the first problem:

As the new carb was shipped without the TPS-sensor, I tried to refit the sensor from my old carb to the new one. The problem is, that the rotating pin that goes into the sensor, turns clockwise on the old carburator, but counter clockwise on the new carb when opening the throttle. If I would refit the old sensor, the reading would be totally off by 180 degrees. Do I have to buy a new TPS-sensor or can I just install the sensor?



I just found 2 different sensortypes online:


-This is from my old carb (with 2 screwholes) My new carb only needs 1 screwhole.


-And this one, which actually has one screwhole and would fit exactly on my carb






Edited by Domnaek

You need the correct sensor for the carb year model.



alright this is slowly becoming a nightmare...


I reshimmed the Intake as I had to change the Intake camshaft because the previous owner shot the Camshaft bearing and the timing was completely off. No wonder it didnt run good. After that I rebuild my new Carb with all the necessary jets. I had to re-use the existing throttle cable as the ones from a 450 didn´t fit. Reassembled everything and it fired up on the second kick on joke today.

And the bike runs awfull... Whenever I touch the gas the bike idles really high, I give it a slight, but powerfull throttlepull and it idles great again. Then sometimes the idle goes really low and it stalls. 


Don´t know what to do now

Are they brand new jets that you put in?

Are they brand new jets that you put in?

 Absolutely yes :)

Reopened the carb, cleaned everything and readjusted the throttlecables as they were a bit too tight. Still having the same problem.

Does anyone happen to know the base-setting for the Idle screw? I dont mean the brass one, but the plastic one for adjusting the opening of the throttle valve?

Your symptoms sound about like a vacuum leak to me.


If it's idling at a reasonable rpm the idle speed screw is set about right.

Yep the idling is "sometimes" in the right range. Im confused on where the Hot start button and where the Chokge button should be?! There re 2 openings of course but both buttons fit bth holes. And I saw a lot of pictures on the web where they used it either way.

A 450 carb does not have a hot start knob on the carb.  That's cable operated, and goes in a vertically drilled bore at the left front of the throttle slide well cover on top.  If you have two horizontal bores on the left side, it's a carb from a 2000 - 2002 YZ426, just a newer issue of the same piece of junk you had already. 


Hot start goes in the forward of the two, BTW.

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