Keihin carb upgrade on Lc4, Honda/Ktm models


I sow someone discuss these things, years ago. So I repost in the hope someone is going/gone through this upgrade recently. In few words here is the problem: I'm doing a MAJOR series of upgrade on a 640 Lc4 enduro , using the 660 rallye (up to 06/08) as target reference. In the end my lc4 will be 90% identical to target in term of functionaliites (and design), including full titanium exaust clones, 4 tanks, totally different lubricating, cooling and fuel systems etc, etc.

Now about carb: there are many fcr 39 (and some 41) around originally mounted on Honda CRF. Will they fit - ONCE rejetted/reconfigured accordingly to ktm specs - on Ktm lc4 640? Or we MUST get only MX mounted on ktm? Please note I dont care about TPS and also ACV. Also 660 rallye up to 2004/5 mounted the 39, and from 2006 the 41... so for sure BOTH of them are ok on this bike, but someone can advice on choosing one or the other?

thanks to all!

I tried sudco inquiry but no info up to now (if you buy carb or buy just spares, did you get by them SAME support)? And

Hey Max, I am not aware of all of the details, but did hear about cable length changes being required.

Perhaps the best advice I can give (for interchangeability) is perhaps to find the same setup off of a 625 SMC (41mm FCR). You won't need to mess with the hot start cables and all that. The TPS can be terminated if desired.

I was considering getting the BST off of my enduro as well (mostly because my SMC just plain RIPS with the FCR - and high flow head).

I would go look at for additional information as well.

Cheers, Robert

Edit: BCR ? BST !

Edited by X-Racer

Hi Robert,

thanks for suggestion. cables and boots are minor issues.. the important would be to "know" if Honda FCR will work perfectly as the Ktm one.. btw.. yes ktmtalk is a goldmine of ktm info.. really.. any info you may gather on this, please post..


The smc660 and smc-sxc625 had a FcrMX 41mm carb with a removable air bell intake to the air box boot .

It looks like the ones you can buy as an aftermarket carb .

The CRF carb hasn't a removable air bell intake , that means it will work good only with a crf or any other bike which uses the same type FcrMX carb .

Any fcrMX which with a non removable air bell intake will work properly on bike which has the same design air boot , that's why you can find these( 3rd generation Fcr) on real low prices and the other with the removable air bell you find them used with prices

like almost a new one from Sudco or any where else that sells carbs .

Hello Smias, thanks to reply and useful info. The boltable airboot adapters seems gone in ktm starting with 08 models where keihin were factory installed, for example in the 660 rallye 08, my reference bike for the upgrades I'm going through. Instead Honda didnt have that boltable adapter since - at least - 05 models (probably they didnt need it). This said, the presence of boltable adapters or not is to me a minor issue, espec considering the COMPLEX mods I'm designing and doing: if I dont find an airboot with correct dia to carb and airbox I just cast/mold one overnite. Comparing honda and ktm fcr and comparing ktm fcr in different years - put aside external cast diameter to airboots, TPS that I wont use and of course jets and needle - I GUESS they are JUST THE SAME. But I still couldnt find a knowledgable person to CONFIRM IT. As soon as distributors got they are not going to sell you a new carb kit but few spares for an used and possibly from a different bike carb, they simply DESAPPEAR (ie: Sudco) or do not reply to Qs (Italy distrib) but just try to push/convince you to buy a new one and they dont like to talk with someone who knows things. I created an online page with schemas and compared pics for this carb upgrade -small part of my MUCH more complex 640/660 rallye bike project. This is a temp pass if you are interested to check: Thanks!

To use a carb without a removable air bell intake you need to machine an adapter to work properly , like this :

FcrMX with non removable air bell intake



and the adapter to fit properly in the air boot :



And how it looks with the adapter on :


Hello, thanks for pics! 1Q... since the "ring" on carb seems to be 10mm+ .. wouldn't be possible directly put - if your boot fit or if you design it to custom fit - the airboot on it?

Btw your custom adapter, how is bolted/fixed on carb? thanks!

It's not mine , found the pics on the web..!

I know for this mod because some people with drz400 bought a fcr carb like this and they use an adapter like this to work it out .

There are some threads on the drz400 section for this .

Here is one i found .

the reason why the airboot looks to me a "minor" issue is because I'm going to design and build overtank airbox and the 4 tank of 660 rallye..... if you want take a look...

if you notice espec in the 08 model the boot goes to the carb 08 w/o adapter... seems to me a direct connection...

You need to go over to KTMtalk dot com, you will find all the info you need there.

I recently installed a FCR39 Gen I (slant slide with external AP) and it was a bolt on with EXC 525 throttle cables.

I then upgraded to a Sudco FCR41 Gen I (same carb as 39 just race model and bigger)

Hi Thumper996, Yes I use ktmtalk to gather info for my 640/660 rallye project, of which carb upgrade is a part. btw, even if 525 engine is very different from 660 lc4, what sensible differences you got by using the 39 and then the 41? thanks, max

Smias, thanks for posting the reference for this adapter. I think I will end up doing something similar... .. .

Can someone tell me where I can purchase one of these?  I am looking to do a similar thing to my DRZ400SM.  



I do a jetting kit for the Husabergs, it is best to see my advert on ebay UK.  this started as being for the 400, then 450, 550, 501, 628 and 644.  then, knowing it would work on all SOHC european bikes: I got it onto the 400, 450 and 525 KTMs.  I've fitted the kit to about 5 KTM LC4s over the years and they have run sweet in the workshop but of course you can't be there with the owner and they can panic a little.  I have left the LC4s till last because nobody races them as such and the back-to-back testing is never compacted and results validated.  


The reason I didn't put a kit up for sale knowing it would be OK was because the LC4s seem to need a Huge PJ compared to anything else and so we needed to kind of believe what we thought we believed....bloody hell!  it REALLY IS true!!!  But anyway, I had someone in this week that did everything I recommended and we've sussed it for sure.  so if any of you care to try it, your welcome to contact me.  


as for this topic, there is a good deal of power to be made by machining the front face of the carb off and fitting a bellmouth.  a bellmouth that would then have a neck behind it for the airbox rubber to fit too.  we fit a 2" stack inside a cone air filter and it makes a lot more power...very noticeable.  back in June, I fitted a blue bellmouth as a stand alone change to the Sidecarcrosser I tune and sponsor and he said the difference was real, especially in the 'meat' of the power. 


with these big engines, the 39/41 are way too small for real performance.  so in an attempt to get more out of the Husey, I have bored one out to 44mm and it runs really well.  however, any of you that have done the big bore mod (to 43mm usually) will know, the centre ring gets out of step with the entry and exit.  


so my next tests, now I have no splutters from the stepped carb: is to put a 1.5mm plate in the carb to lower the centre ring.  the centre ring is also weak now of course because it is so thin.  I'm going to 'try' (as the engineer has the last say) and get these centre rings made with as much precision and meat left on them as I can so they aren't so frail but it should mean a good 44mm carb.  I just need to know that lowering the centre ring and the lower half of the carb is not going to run crap.  one step at a time right?  


just to let you know what kind of power they release, I have a Husaberg I tuned in the Norwegian Kross-kart championship and he uses the big Mikuni.  he swears it produces over 90bhp.  now I could tell you what he really claims but I find it impossible to believe!  so let's all just say that he "claims 90bhp" and it HAS to be the big Mikuni as I know the rest of what makes the motor go!


Essentially, a jetting kit, inside a 44mm carb, with a 2" stack and a cone air filter (OK, maybe not on the KTMs but necessary on the Huseys) will produce a huge leap in performance.


kind regards



Taffmeisters UK

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