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View Results: Keep the FCR, or return to the BST40?
Keihin FCR - This is the future, get used to it! 6 75.00%
Mikuni BST40 - Oldie but a goodie! Don't fix what aint broken... 2 25.00%
Voters: 8. You may not vote on this poll

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Old 05-04-2015, 03:52 AM   #1
hell4leather OP
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Eh? Carb: Remove FCR, return to BST40? '02 KTM 640 Adventure

Hi folks,

I'm the proud new owner of an '02 KTM 640 Adventure and loving it! BUT... It's running a bit rough, and it seems to be carb related, leaving me with an unusual question to consider :

Do I try to remove the existing untuned aftermarket Keihin FCR carb and put the original Mikuni BST40 back on (back to a 'known' good state), or try to tune the FCR myself from a zero-knowledge starting point?

I know the thread question sounds crazy (going backwards?) but please hear me out, and then feel free to dish out your opinions whatever they may be

I bought the bike recently, and it came with several modifications, of which the most significant was an essentially brand new Keihin FCR carb (don't know if its a MX, 39 or 41). The previous owner had it installed but not tuned in any way, then immediately afterwards stopped riding the bike for about a year, then sold it to me! The only thing I know is that the people who installed it told the previous owner that it would need to go on a dyno to be correctly tuned, that the cost would be about $400, and that this option was refused.

The problem is that the bike is popping a lot on deceleration (occasionally with flames coming out), and if decelerating really fast it will sometimes stall even with the clutch in. It's also stuttering a bit when cruising at constant speed, but stops that with any changes to the amount of acceleration provided.

From my research, all of the above symptoms point to it running lean, but given that I have zero experience in tuning carbs and would rather not spend 400 bucks on it being done for me, I'm considering my other option, which is to just install the stock Mikuni BST40 that the bike originally had, and hope that it either runs well as-is, or that I can use the tonnes of BST40 / KTM 640 info in this forum to tune it properly.

I realise that if I re-install the BST40 (in the box of spares that came with the bike) that it will probably need a rebuild after drying out for at least a year, plus a presumed lifetime of use up to 27k kms... Regardless, it seems like it might be an easier option!

I should add one more thing : I don't want/need the best possible performance in the bike! I just want reliable smooth delivered power. Thank you for your opinions!
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Old 05-04-2015, 04:46 AM   #2
tHEtREV
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I have an 02 and put an FCR on it, but the fuel screw is chewed out and I switched back to the BST, I will be fixing the FCR very soon as it is much more fun to ride with it.

I got a jetkit for a 525 to put in mine, and following the instructions it was pretty close to being spot on (except for the fuel screw that is), but I only bought the Dynojet kit, apparently the JD jet kit is a lot better and pretty much spot on.

So my advice is work out which carb you have (39 or 41), get a JD jet kit, and call it good.
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Old 05-04-2015, 06:43 AM   #3
Kawidad
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Also, does your riding style and intended usage lean more toward the FCR or the BST?
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Old 05-04-2015, 08:51 AM   #4
Grreatdog
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I have zero complaints about the BST on mine. But mine is also running an SXC competition muffler, opened airbox and jet kit. One of the PO's went to a lot of trouble dialing in that BST. So I would be tempted to go back to stock and sell the FCR for some extra cash. But that is no guarantee. As you said, you are probably still looking at some rebuilding and tuning with that option.

So tHErEV makes a good point. Investing maybe $100 in JD jet kit and fuel screw for an SXC Keihin carb (assuming your's is the same) might be an eye opening experiment. Those guys make jetting pretty easy by giving you a baseline setting that will likely be 95% correct out of the box with simple instructions to further tune. I ended up leaving my MXC at their baseline. Same for my 640.

I don't see a BST kit on their website. Maybe my 640 kit is a different brand. I haven't dug through the spares box in a while and my memory sucks. But JD still sells the SXC Keihin kit.
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Grreatdog screwed with this post 05-04-2015 at 08:59 AM
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Old 05-04-2015, 10:10 AM   #5
kevsta
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lol, I have the BST with an SX competition muffler and mine has always done this:

Quote:
It's also stuttering a bit when cruising at constant speed, but stops that with any changes to the amount of acceleration provided.
and as it happens in pretty much every gear it leads me to assume this is just what these engines do. ie they're designed to be ridden hard, not sedately putted around town in traffic or that steady speed nonsense

..other than the absurdly lumpy vibrational bag-of-spannerness if you try and "drive" it sensibly, its everything I ever wanted from an engine

"Ready to Race" and all that..
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Old 05-04-2015, 11:39 AM   #6
matty86suk
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I swapped a bst from a dr650 onto my 400 rxc and can`t be happier, mind you I was replacing the Qwiksilver 2. I like CV carbs on my dual sports, easy to tune, reliable, adjusts a bit for altitude, smooth power delivery, etc...
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Old Yesterday, 04:36 AM   #7
hell4leather OP
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Thumb

Folks, thank you all for your quick thoughts and advice!

It sounds like the JD Jet Kit is an option that might make the existing (Keihin) setup a possibly workable solution. (Thanks @tHEtREV and @Grreatdog!) I'm still going to have to do some thinking... maybe will try some initial fuel screw tuning to see if it improves at all. If it does, I'll invest in the kit and see if I can make it work. If it fails, I'll sell the FCR plus the jetting kit and probably still come out with some cash after rebuilding the BST as the last resort!

It's a pretty interesting point that youre making @Kawidad and @kevsta about riding styles. I'm probably going to be riding it less like it should be ('ready to race') and more like a speed-camera fearing good citizen (with occasional offroad side trips!) Im guessing that doesn't line up with the FCR's 'performance' reputation. Whatever carb I use in the end, I'd definitely be interested in something that might help with the vibes...which are currently pretty awful!
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Old Yesterday, 07:02 AM   #8
Kawidad
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My 640 Enduro was the definition of "paint shaker". However, the carb once dialed in was actually pretty good. I did have to rebuild it with parts from Derek at MotoLab (here on this site). Once that was done and with proper jetting, it was surprisingly good.

Something to consider, is the FCR can be very finicky. One thing that you must look at and is often overlooked is the slide. The FCR has a 2 piece slide (more actually, but you get the idea). The slide has a vacuum port cover that is made of magnesium and has a small rubber seal around it . This part is critical to the proper operation of the carb. This vacuum cover will crack and those cracks sometimes are almost microscopic, so they have to be carefully inspected. This was fairly common on the early FCR models, but doesn't seem to be as common now. Since you apparently have a donor FCR, then this should be inspected, or else it could drive you nuts without a proper explanation.
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Old Yesterday, 05:34 PM   #9
Droptarotter
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Get the JD jet kit for the FCR!

That's my vote!
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Old Yesterday, 06:24 PM   #10
trailrider383
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I don't ever see mentioned that the carb has been cleaned. You mention the previous owner let it sit for a year. You can't solve a carb problem by trying to tune a dirty carb. The carb needs to be cleaned by someone who knows what they are doing and then the tuning can begin.
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Old Today, 03:53 AM   #11
hell4leather OP
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Quote:
Originally Posted by trailrider383 View Post
I don't ever see mentioned that the carb has been cleaned. You mention the previous owner let it sit for a year. You can't solve a carb problem by trying to tune a dirty carb. The carb needs to be cleaned by someone who knows what they are doing and then the tuning can begin.
Thanks, a very good point taken! I'll add that to the top of my list and get to it!
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