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-   -   2 questions for the gurus re: FCR-41 carb on a 640 (http://www.advrider.com/forums/showthread.php?t=43819)

PASSMORE 06-06-2004 05:47 PM

2 questions for the gurus re: FCR-41 carb on a 640
 
Hi all,

Got a problem and a question for those more experienced than I... I just got my bike from service and they properly jetted my carb (stock exhaust FYI). Test ride at the shop went swimmingly (read awesome!). Now, the second I take it off choke it dies immediately. I really do not want to take it all the way across town, so I want to see if it something I am just overlooking... IDEAS, SUGGESTIONS?

Next, is there a way to wire the stock choke (handlebar mount) on this carb rather than fiddling for the choke nob up underneath the tank?

Thanks in advance!

Michael

Mack 06-06-2004 06:26 PM

From what I am told, a properly jetted FCR often does not require any choke, just some twist to operate the accelerator pump. The more sophisticated fuel circuits seem to handle the mixing much better then the BST, hot or cold. I would first try turning up the idle a hair, or more, see if it keeps running. Remember the FCR has not one but two air adjustment screws. Both cover the 0 to 1/8 throttle, one IMS (IIRC) has the strongest effect at zero tapering to 1/8th, the other increases its effect as you get closer to 1/8th. (One is under the carb, the other is in the intake bell.)

As I understand it, the Mikuni choke plunger can be removed and installed in the Keihin, no first hand on this. Let us know what you find. Good Luck, Mack

PASSMORE 06-06-2004 07:09 PM

Mack,


Thanks for the tips. I will fiddle with it some more this evening.

Michael

PASSMORE 06-07-2004 10:04 PM

Problem solved - slight gap allowing air where the carb mates up to the block. All is VERY well now... Holy cow! what a difference the properly tuned carb makes!!! A really significant dose of power!!!

Mack 06-07-2004 10:11 PM

Great to hear you got it fixed. Was the intake boot swapped out during the carb change? The BST and FCR boots look pretty similar, but close examination will reveal that they are different. A leak up stream of the carb is bad, but minor compared to a downstream leak that can possibly cause lean running, and engine damage. Now you can tell us about all those power wheelies, just do not dump all your oil in the airbox now. :thumb

PASSMORE 06-07-2004 10:40 PM

mack,


unfortunately, being a used bike with the carb already on I do not know if the boot was swapped or not. Basically it was just not tightened down well enough and the clamp "head" was in a position as such to be able to be jostled easily. I did not see it right off, but dick did as soon as I got it back over to him... I am still not so sure about all of the power wheelie claims I read in "whatever" gear... Right now I do know that it is exciting again to ride - acceleration wise... If I was doing it all over again, i would definitely instill this carb on this bike.

I am looking forward to seeing what an exhaust will do next!

Michael

gozell 06-08-2004 02:01 AM

Hi Mack,

Just came across a KTM KEIHIN FCR 39 from a 520 EXC 2002

This would also suit a 2003 640 Adv (with some mods)?


Grtz

gozell 06-08-2004 02:03 AM

opps, suppose i should do a search first

http://www.advrider.com/forums/showt...ght=keihin+fcr

Mack 06-08-2004 02:21 AM

Your certainly headed in the right direction. I bet either would work, I went with a 41, but I still want to try a 39 as well just to see the diff, dyno curves, etc. My guess is the 660 Rallye gets by with a 39 as IMHO its motor is a better matched to this with the high compression, they really need the low end, dual exhausts, and that garage the rallye calls an airbox, etc. I suspect a FCR41 fitted with a Powernow intake will operate much like a 39, that is my plan. Still a properly priced and well sorted 39 might just be the ticket. Cheers, Mack

PS. I would think that it might make a difference if it is a pre-03 or otherwise.

BlownCJ 06-08-2004 06:08 PM

Quote:

Originally Posted by Mack
Your certainly headed in the right direction. I bet either would work, I went with a 41, but I still want to try a 39 as well just to see the diff, dyno curves, etc. My guess is the 660 Rallye gets by with a 39 as IMHO its motor is a better matched to this with the high compression, they really need the low end, dual exhausts, and that garage the rallye calls an airbox, etc. I suspect a FCR41 fitted with a Powernow intake will operate much like a 39, that is my plan. Still a properly priced and well sorted 39 might just be the ticket. Cheers, Mack

PS. I would think that it might make a difference if it is a pre-03 or otherwise.

not true got an fcr 39 with the tps and hot start off of ebay from a 03 450 exc well see how it does on the 03 adv with a rallye cam and akra

Barry

Mack 06-08-2004 06:31 PM

Barry, your confusing me, what are we talking about? All the curves I have seen for the 03 VS. earlier bikes, show the power is moved up the powerband a ways, much more like a street bike then the earlier bikes lower rpm peak hp. So if that is even what we are talking about, my comment regards the fact that it hard to peg what would be best on a free flowing revvy 03 motor! It seems to me for mixed DS use the 39 with the ADVs undersized airbox might be chokeed down a bit for on the road use, though it should work really nice down low offroad. It will be interesting to see your dyno runs. Cheers, Mack


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