FCR 41 vs FCR 39?

[Research Rat]

So it seems like running an fcr cures most of the stock xr650l carb throttle response issues.

From the posts here, it sounds like most are running FCR MX 41, which are stock on many KTM, Husaberg, and Husky machines. There are a few FCR MX41's on ebay, around the $350 range.

I've also noticed fcr 39s on ebay, where these are typically taken off drz400s. The cost on fcr39s is about $100 less costly than fcr41s.

I'm wondering:

A. Which is the better carb for an xr650l? Is there any difference?

B. If relatively no difference in performance, will the the FCR39s will fit on xr650ls?

Thnx.

-Scott

[Kenzo]

FWIW, i'm still happy with the R to L carb swap for less than $100...

...from a dead idle and stop u can still bog it if u go WOT instantly but other than that it has performed very well...i'm looking forward to see how it does with a HotCam

hopefully i get a chance to ride an L with a pumper to compare but i doubt the difference is worth $200-$300 more...

here's a link https://www.thumpertalk.com/forum/showthread.php?t=114439&page=7&highlight=carb+swap

[Research Rat]

Kenzo, thanks for the input.

There aren't too many xr650r carbs on ebay right now and the one is already priced over $100.

How is the xrr carb different than fcr?

Also-do you you have any knowledge on 39 vs. 41 fcr carb?

-Scott

FWIW, i'm still happy with the R to L carb swap for less than $100...

...from a dead idle and stop u can still bog it if u go WOT instantly but other than that it has performed very well...i'm looking forward to see how it does with a HotCam

hopefully i get a chance to ride an L with a pumper to compare but i doubt the difference is worth $200-$300 more...

here's a link https://www.thumpertalk.com/forum/showthread.php?t=114439&page=7&highlight=carb+swap

[martinfan30]

I would go with the 41mm FCR for the bigger 650 engine. A 39 would work, but the 41 will be feeding the big thumper what it needs! I am actually considering having my non-MX FCR bored out to 43 soon.

The R carb is a better carb than the CV. It's a round slide, non-cv carb. Still no pumper.

Yes the pumper carbs give incredible response.

[BWB63]

I have ridden with MartinFan and the trueth is very few "L" guy's ride the "L" the way he and Denn10 do. Not a put down on the "L" guys (well not my usual). I do believe Kenzo is right for 90% of "L" riders and for those that do 70%+ street it would be the best porformance for the $$$ and still keep good fuel milage. The stock "R" carb. was hard to get $85 on ebay when I sould my two there. Kenzo should be knowns as the "R" Carb. to "L" bike guy:thumbsup:

Now if you ride mostly dirt with your "L" and ride it like you should have bought an "R".....go buy an "R":smirk:

[martinfan30]

I have ridden with MartinFan and the trueth is very few "L" guy's ride the "L" the way he and Denn10 do. Not a put down on the "L" guys (well not my usual). I do believe Kenzo is right for 90% of "L" riders and for those that do 70%+ street it would be the best porformance for the $$$ and still keep good fuel milage. The stock "R" carb. was hard to get $85 on ebay when I sould my two there. Kenzo should be knowns as the "R" Carb. to "L" bike guy:thumbsup:

Now if you ride mostly dirt with your "L" and ride it like you should have bought an "R".....go buy an "R":smirk: :ride:

I'm looking Bruce! Wifey has alternative motives though.

[Research Rat]

My bike is slowly becoming offroad dedicated, so better throttle response is definitely necessary.

Why would I want an R, I have electric start and a sledghammer reliable engine, turnsignals and a horn!

-Scott

I have ridden with MartinFan and the trueth is very few "L" guy's ride the "L" the way he and Denn10 do. Not a put down on the "L" guys (well not my usual). I do believe Kenzo is right for 90% of "L" riders and for those that do 70%+ street it would be the best porformance for the $$$ and still keep good fuel milage. The stock "R" carb. was hard to get $85 on ebay when I sould my two there. Kenzo should be knowns as the "R" Carb. to "L" bike guy:thumbsup:

Now if you ride mostly dirt with your "L" and ride it like you should have bought an "R".....go buy an "R":smirk:

[martinfan30]

My bike is slowly becoming offroad dedicated, so better throttle response is definitely necessary.

Why would I want an R, I have electric start and a sledghammer reliable engine, turnsignals and a horn!

-Scott

Haven't ridden an R yet? Don't....

[BWB63]

I got the starting price of $85 a peice for the stock "R" carbs a couple of years ago but, if others jump on Kenzo's idea they will only go up in price. I could see them going for as much as $150 and still being a good deal. Kehin 40mm round slide does everything but, go from almost stall to full throttle in one wack. That doesn't happen inless your racing or drop into a big ditch and need the front end up now to lift the tire for the fast coming other side......I am not pushing the "R" carb as the answer to the "L's" CV carb for everyone but, for most it would be big improvement for only a third of the cost of a real carburetor.

[Kenzo]

I have ridden with MartinFan and the trueth is very few "L" guy's ride the "L" the way he and Denn10 do. Not a put down on the "L" guys (well not my usual). I do believe Kenzo is right for 90% of "L" riders and for those that do 70%+ street it would be the best porformance for the $$$ and still keep good fuel milage. The stock "R" carb. was hard to get $85 on ebay when I sould my two there. Kenzo should be knowns as the "R" Carb. to "L" bike guy:thumbsup:

Now if you ride mostly dirt with your "L" and ride it like you should have bought an "R".....go buy an "R":smirk: :ride:

well...i ride 90% offroad, i hold my own but my fuel milage sux with the R carb and i haven't even cam'd it yet but i do make piggy squeal...

the funny thing is, when i biff it...it does not pee all over it's self like a skeerd puppy...no problems starting it ether...i guess that's an R thing?

i'm planning on comming out to SoCal in the near future...we're gunna buy up many of the illegal/outlawed R's and bring them back to the Republic of Tejas where we like gasoline burners...hell i might even wait until Bruce throws in the towel and starts riding a Husky...i cud part out all of that bling and make a small fortune

[martinfan30]

I got the starting price of $85 a peice for the stock "R" carbs a couple of years ago but, if others jump on Kenzo's idea they will only go up in price. I could see them going for as much as $150 and still being a good deal. Kehin 40mm round slide does everything but, go from almost stall to full throttle in one wack. That doesn't happen inless your racing or drop into a big ditch and need the front end up now to lift the tire for the fast coming other side......I am not pushing the "R" carb as the answer to the "L's" CV carb for everyone but, for most it would be big improvement for only a third of the cost of a real carburetor.

Great point, especially for those of us on a budget.

[Kenzo]

Great point, especially for those of us on a budget.

yup...i got the R carb, cables and throttle assemblies for under a $100 shipped

i bet there's somebody on this board that might be willing to sell theirs...

haven't even tried the carb with the HRC needle yet...i guess i need to pull the trigger on the cam and start a new project

[BWB63]

I think the stock Kehin carb on the XR650R worked 100% as good as most pumpers if the pump isn't needed and most of the time it is not. It is those few times that makes the bike jump from almost stopped to roaring in less then a wink that you just get addicted too! Well and it starts easier, and more adjustable and then there is the Edelbrock that has it's own nitch that keeps the air fuel mixure better for a wider elevation change. They all do a job and some better then others.

[Denn10]

I'm looking Bruce! Wifey has alternative motives though.

YEAH well ive already found one:moon: NANNY NANNY BOO BOO:banana:

AND dont PM me and ask for pics and such cuz i aint spilling NO BEANS, after last time when someone swiped it up from under me BASTARDS:banghead:

[Research Rat]

Update:

2004 CRF450R is equipped with a keihin fcr 40 carb.

http://www.hondabikes.net/honda-crf450r.html

Should be direct replacement for xr650rs.

-Scott

[Denn10]

ive looked at some of the FCR MX 41s and they are pricey for sure, for the money i would go with a R carb on a L if money is an issue as you will get great performance enhancement for the money. If money is no worry andyour looking for max potential with down the road mods also then get a FCR41 or a Keihin 42 from XRs.

Im sure that 40 off the 450s would be good alternative also.

[Research Rat]

I spoke with Sudco today re 39, 40, 41.

1. All fcr mx models have the same dimensions. IE no fuel volume capability difference.

2. 39, 40, 41 is the size of the internal venturi opening.

3. When Sudco dynoed different carbs on an xr650l, there are smallhp difference between 39, 40, or 41 mm sized models. "Negligable differences" was the term used.

4. The 39 will produce better down low reponse than 41, 41 will conversely produce better high end response than 39.

5. The Sudco contact remarked that he'd seen good results with using the 39 mm on xr650ls.

The Sudco contact recommended thinking less about engine size in relation to venturi size needed, rather than relationship between hp and venturi size. His rationale, crf450r with a 40 venturi opening, makes more HP than xr650l with the 41 venturi opening. I still don't understand this as 450 has 200 less cc's than the xrl and consequently would require a smaller overall combusatable mix.

All the above aside, our bikes need a measured amount of fuel/air, which is based on engine size. I think the 39 mm with rejetting can deliver the "right amount" but the question is can it deliver at the same rate of speed as what's needed. I'm not sure about that yet.

Thoughts?

-Scott

[BWB63]

To start with Honda forgot to put any lobe on the "L" cam and what compression? The "L" is so detuned from what it can safely put out without any difference in reliablity it is a joke. Get a cam one the "L" and 10:1 compression and you will get a lot more power out of the top RPM's. You top rpm change with a carb change will never be tons for just a couple of mm but, the change will be 6000+ but, your right even the differnce between a 39 and 41 will be small for a stock "L".

[sbabs]

Is the FCR41 a direct bolt on or does it require a lot of mods? What about cables? Do the XR650L stock cables work?

[martinfan30]

Is the FCR41 a direct bolt on or does it require a lot of mods? What about cables? Do the XR650L stock cables work?

Mine was a SUDCO carb, it came with new motion pro cables and it bolted right up.

I think the barrel ends are different.