450 carb on a 400?

I recon the 450 carbs are bigger than the 400's. Does it fit? Anyone know the size of the inlet on the 450 head? My 400 head is ca 49mm on the head inlet tubing where the rubber boot sits.

so, can I mounth a 450 carb on my 400 together With a 450 rubber boot? Does both heads have the same Diameter on the inlet?

thanks on advance! :ride:



Edited by Chas_M


I appreciate the link, that is awesome to know on an easy fix. Thanks again.

Thanks MountainRider! (y) :thumbsup: 


I didn't realize that this post was in the Yamaha forum at first. In any case, wouldn't the 400 carb be the same or similar carb as the 39mm FCR in the 450?

I didn't realize that this post was in the Yamaha forum at first. In any case, wouldn't the 400 carb be the same or similar carb as the 39mm FCR in the 450?.


I asked a seller of a 450 carb on ebay to meassure the Diameter on the flange going into the rubber boot towards the engine. He answered 55-56mm.. And because the 400 carb is 39mm inside and 46-47mm outside, I started to think maybe there is different Outside diameters on some of the keihin 39 carbs, to fit different bikes... 

Thats why I asked this question.!

But now I've got confirmed in another forum that a 450 will fit, and does have the same diameter as the 400 carb. So the seller must have missed by 10 mm.. :D

So really effing dumb noob question here; but is the inlet and outlet of the internal throttle body of the carb larger to let more air in on the 450's carb? Kinda can't tell from the picture, though it says it is a direct fit for 400/426's.

Yes. and fuel. and more power. and a better throttle repsonse

Man, I really appreciate this thread. I really want to do the Yz450 cam swap to get rid of the effing decomp. It disheartens and frustrates me riding trying to get that c8nt started in the woods. While everyone else just kicks and ready to go.


Eventually I will do the 450 carb swap and full exhaust, but least of my concerns right now.

Can you use the Yz450 Intake and exhaust cams as a easy and cheap hotcam for the 400/426?

You can get yz timing using the stock exhaust cam or you could put in a wr450 exhaust cam that will help with the kick starting since you would be getting the auto decompression I believe the link I posted earlier actually mentioned doing this.

So looking at the picture more indepth, the WR450 carb is on the left and and a Wr400/426 is on the right. Correct me if I am wrong, but the inlet diameter looks 20% smaller than the 400/426 carb. I am not saying I am skeptical on fuel air mixture, but the supposed power it brings over the stock part with more efficiency to the engine with better throttle reponse.

wr450 carb.jpg

The 400/426 carb is a much more crudely done designed carb. It has notorious flat spots and wear points....but does make great power.

The WR450 carb can be brought up to 2008 specs with a few parts. It can be set up to be FI quality no problem 

The YZ450 carb 2006/7 is a bit better with the exclusion of the ACV, and a different needle jet and jet needle.

Does it help on less stalling on steep inclines?

Does it help on less stalling on steep inclines?


Depends on why it is stalling


If you mean the angle, yes


If you mean low rpm , maybe


If you mean increased torque, no.


If you mean better throttle control at partial openings, very much yes


If you mean gear, clutch control, and general riding skills, then no!

I just want to share with you the fact that i've got a 2005 wr450f carb in my mail a few days ago, got it mounted where it should be -on my bike, a -99 wr400f. Just had to turn  the air/fuel mix screw a bit anticlockwise to about 2 1/2 turns out, to give it some more fuel. Now the bike is running flawless without any signs of hesitation or bog anywhere on the throttle.!
The carb fitted right into the Stock rubber boot to the engine head! I was told that the throttle wires is different on the carb side on the 450 compared to he 400's, so the seller included the wires in the box! Perfect! :thumbsup:  :ride:

Cools! I just bought an '07 Wr450 carb w/ cables off of Fleabay and going to install it over the winter with other mods I prefer to do to my Wr426f.


It also turns out my forks are leaking; so going to a Racetech Springs and gold valve kit, rear spring is shot, so that is also getting a Racetech spring this winter.


Also in the works a Hot Exhaust decomp cam, because trying to start the bike in the woods fawking suckass without the auto-decomp. Plus adding a full FMF Q4 exhaust.


I just like tweaking and keeps me busy in the suckass Indiana Winter. It is just what I do!

After changing the carb, I don't have to stand on the bike to kickstart it anymore! It starts so easy I can do it sitting on the bike now, everytime on the first kick. Even without the choke engaged, and running dead steady idle! Feels great man! (Y) :)

Edited by Ro9

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