Which carb is best on LC4 640

Discussion in 'Thumpers' started by losiu, Sep 19, 2006.

  1. 00jmc

    00jmc n00b

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    Sorry to jump in here guys but I need some help w/the 41 on my Duke. The bike is new to me and the previous owner put the 41 on this past winter. Does the 41 have a choke or is there a secret to starting w/out it? The previous owner just said it didn't have one. Please help, hard to start in this 30 degree:D weather!
    #61
  2. lvdukerider

    lvdukerider Las Vegas Nevada

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    It has a choke knob on the upper left side of the carb, its kind of hidden by the tank. I used the old choke plunger off the BST40 by simply cutting and fileing off the brass protrusion in the center, so I could retain the handlebar choke control.
    #62
  3. Flyin Brian

    Flyin Brian Been here awhile

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    I haven't ran across Creepers update on his FCR install, but I'm getting closer to going that route. Last year when I first got the bike it bogged out after going through minor whoops (really annoying). After reading through all the info on this site. I got the carb all cleaned out and adjusted properly over this past winter. Well the bog was gone, and I was happy. I figured the FCR would be a performance addition only, not a bog remedy. Well today the bog came back. Of course it happens when you really can't afford it. I was starting up a hill climb and had to go over some whoops at the start of the hill. Well the damn bike just starting sputtering and died. I had to let it sit for a few minutes before it would run correctly again.
    So in the short term whats that best way to get the bike running properly (besides waiting) after suffering from the bog? Long term..... I'm looking for a FCR.

    Creeper..... How's that FCR doing for you? Elevation changes an issue?

    On another note.. .It was great day riding. The conditions are perfect out here in the high desert!
    #63
  4. losiu

    losiu wheelie addict

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    It is possible to start a bike that has a pumper carb without using the choke by simply whacking the throttle open a couple of times before you start it. The pump will squirt some gas where it's supposed to go and the bike should start. I believe the choke would simply work better because it's designed to help start cold engines and it definitely IS on the left side of the carb.

    Depending on the carb model, there'll be one black knob that you pull or two - black and red. I've never had an FCR but the red one should be for "HOT START".

    The BST is a total PITA sometimes. I have no time to do much offroading but I know from experience that it's simply unpredictable. Sometimes it works fine and sometimes it doesn't. Properly re-jetted and set up it probably could be comparable to pumper carbs but it needs to be said: It'll never be/beat a PUMPER CARB.

    I've ridden several bikes with pumper carbs since starting this thread and I know that the bikes tend to be smoother and even if they don't provide jaw-dropping performance in comparison with the BST, they are fully predictable and don't cause problems when off the road.

    I think I'd choose an FCR39 for my riding style. Thank you all for posting your experiences :)

    Losiu
    #64
  5. humdinger

    humdinger bp

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    After a long wait, I've got the FCR 41 and full Akro system installed on my '05 640 Adv.
    Totally different bike. The engine is super responsive and silky smooth. So much so that the bike 'feels' 20 kgs lighter even though I think I only dropped about 6 kgs. Stripped all the SAS crap off and 'bunged' the hole between the exhaust ports. No more backfires!
    This is how she's jetted and set up:

    170 main
    45 pilot
    Fuel mixture 1 and 3/4 turns out
    Blue needle (JD Jetting) clip on 5th down from top
    Didn't adjust pump duration; left stock at 3 secs.
    Elevation: 0 to 4000ft
    75% + humidity
    22 degrees centigrade (Average; varies from 15 to 26)

    The big girl really pulls hard from pilot all the way to WOT and I was surprised how much better it 'torques' along at low revs, especially when doubling. She's a tad rich down low but seems spot on every where else.
    I can start her cold with a couple of short twists from the throttle and hot starts just require a short stab on that magic black button.
    Overall, the installation was pretty easy, even for some sub-standard grease monkey like myself. The Akro system was a little close to the frame in a couple of areas but otherwise a lovely fit. I routed the new (slightly shorter) throttle cables down the RHS of the backbone and simply removed the existing connection plugs from the wiring harness that serviced the BST. I didn't have to re-route any wiring which runs top RHS of the new carb. All fuel lines and overflow tubes run neatly away from the exhaust.
    The stock gearing of 16/42 is perfect for the road but I'd like maybe a 45 tooth rear for the scrub. Haven't looked at fuel consumption yet but I'm thinking it will be more than that of the BST.
    Was it worth the money? That's debatable to some but like the saying says:
    "It's a long time, dead!"
    I can honestly say that the 640 feels like a fire breather now with a new lease on life!:thumb
    #65
  6. potatoho

    potatoho Cheese and Rice!

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    Personally I feel you'd need to hack on a pumper to make it equal a CV carb :D

    When I went back to the BST I hooked up an air-fuel meter because I wanted to get to the bottom of things. Sure enough, there was a mammoth dip right off-idle, going from about 12.5:1 pegging to beyond 15:1. The motor sound was hardly even noticeable, it only sounded slightly rough. However it was very weak. So at low rpm the motor tolerates a really wide range of mixtures without giving much of an indication to the rider.

    My needle was worn and it caused the taper to be completely incompatible with the motor. If you adjusted it for mid range, it would be too lean off-idle, and if you adjusted for off-idle, it would be far too rich in the mids. When I put in a new needle, I was able to adjust to the normal clip positions and not have those anomolies. The off-idle was like SNAP!

    So unfortunately, I think the CV carb tends to wear out the parts a bit more. My needle jet is also obviously ovalled, which is another thing probably throwing off my mixtures from the accepted settings. I haven't had any complaints with the performance once I got the breadth of the throttle mixtures worked out.

    The real fun will be experimenting with airbox flow and exhaust flow and how it relates to realworld power. It is kinda cool watching the mixture change when you add the side cover, and then the seat etc.
    #66
  7. Some Dude

    Some Dude what attitude problem???

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    I had a 98 620 SX that had the Delorto piece of crap on it, it was a pain to hardstart and impossible to keep running consistantly from clogged idle passages to you name it. I ditched it after being stranded on the side of a mountain in single track and not able to restart the bike after it bogged to a stall.

    I then installed the FCR 41mm kit from sudco, in my discussions with them I had asked specifically about hot start operation on the kit. Well to my disappointment they do not offer an FCR kit for the LC4 with hotstart as a feature. So I then threw a fuss and they sold me an FCR 41 MX style carb at a hefty discount. This was like heaven. The bike was wheeling good in 2nd gear and a slight pull up was needed to loft it in third, with the FCR MX it was able to pull a wheelie all on its own in 4th gear on power alone. It turned it into a monster that was easy to hot start when the time came. After riding a few LC4's with both the BST and Delorto, I wouldn't own one without the FCR.
    #67
  8. Lornce

    Lornce Lost In Place

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    Wow.

    This is the most interesting post I've seen in ages. Would love to hear more about your mixture analyzer as you have time to post.

    :thumb :thumb !!!

    Lornce
    #68
  9. potatoho

    potatoho Cheese and Rice!

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    I started this thread:
    http://www.advrider.com/forums/showthread.php?t=209038

    The dial-a-jet is a bust so far. The BST was running good on the last ride. I think the exhaust leakage I suspected had more to do with my motor's freak-out response to an anemic mixture in the mids. Seems if I get below a certain ratio it will freak out. The needle was eccentric and making it hit a weird mixture spot. With my new needle I was able to start bumping it up and it settled down and stopped with the weird anomolies.

    I need to increase the rear suspension on the bike before riding it again. The motor ran fine however.

    I should mention that trond tested a BST and an FCR with the same meter. He did settle on the FCR, so my statement about the BST being better than a pumper is not shared. I just think it has more potential if you were to adjust the damping and spring rate, or if we had some more needles to work with. Also the BST isn't the only CV carb available.
    #69
  10. Flyin Brian

    Flyin Brian Been here awhile

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    Do you have a parts list for just the FCR install? Did you purchase a new carb, or find a used one from another bike? Thanks in advance.
    #70
  11. rideaholic

    rideaholic Been here awhile

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    I have the quickslobber on my 97 RXC 620. I have torn it down 5 times thinking I could fix it's problems. I don't think there is a snowball's chance. The problem is anytime I hit a jump or bump the engine dies. Can be very dangerous at times. It has a K10 jet and a KTM MX exhaust. I have other jets, but have not done a plug check yet. Any suggestions on a fix or should I just spend the $$$?

    Thanks
    #71
  12. rideaholic

    rideaholic Been here awhile

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    I adjusted the float level and all seems fine now. I guess I will see. I am still buying a new carb though.
    #72
  13. ridelikeistoleit

    ridelikeistoleit slappy the hamfisted one

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    hey,can't say much but I changed my 96 rxc 620's edeljunk[stick to what you know,car guys]to a fcr39 [paid to have installed,tuned,bought new domino twist grip,and modded air boot and don't think it was close to 1100,but was'nt cheap]and it was a night and day difference!always been good at wheelies but now I'm ggrreeaatt![okay,a little goofy too]:evil throttle modulation is great and control exact throughout rev range-bottom to top!I dont know bst's or dell'oto's but this thing aint broke now so I ain't fixin it!:freaky
    #73
  14. rapiti

    rapiti IOR Veteran

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    I wish I had read this before my latest round of ebaying.

    I just bought an FCR 39 off a YZF450. I was unaware that physical dimensions varied so much. I think I got the wide one[​IMG]

    Any advice? Read the bleeping thread?!
    #74
  15. rideaholic

    rideaholic Been here awhile

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    I am not sure if that's the same style carb as I just bought, but the FCR I just bought and intalled was off a CRF 450. My gas inlet is different and I have a black plastic cover. I put it on a 97 RXC 620. It was a snug fit, but fit it did. I had to make an aluminum adapter to extend the length a little and also changed the screw that held on the cover for the cables. It started very easily. The jetting is close, but I plan on making it perfect as soon as the weather around here in Pittsburgh, PA starts to cooperate.

    Maybe post some more pics of other angles of the carb. If you can't get the carb to fit, just sell it on ebay, forums, or craigslist. You shouldn't lose too much $. Good luCK.
    #75
  16. rapiti

    rapiti IOR Veteran

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    I haven't paid for it yet. I contacted the seller to see if they would sell to second bidder, and I pay them the difference in our bids. Worst case, I will need to sell it again. Need to read more before bidding!
    #76
  17. rideaholic

    rideaholic Been here awhile

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    I paid $100 shipped. It included throttle assembly, cables, adapters for manifold and air box, and clutch perch with hot start lever. I think I got a decent deal. I found it on SM forum.
    #77
  18. madmac

    madmac Adventurer

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    My katie is a 2004 640 LC4 Enduro with the "high flow head", it had the standard BST and a SR-Racing silencer. Apart from subideal wheelie and jump performance it ran quite well. I got a used FRC 41 on ebay in top condition last fall, which someone had run on his supermoto competition bike and had it installed and tuned by Griesser (KTM dealer in Rosenheim, Germany who does a lot of sport bike preparation). They also put the bike on a dynamometer. It has now 59,2 hp at 6585 revs and 68,8 Nm at 5510 revs. Much more important for me than the paper performance is the way the bike now reacts to every "twist of the wrist" now, the bike is lots more fun to ride now than with the BST. I can really recommend this kind of setup and the new LC4 has lost its chance to be bought by me. But- if you spend the money on a flat slide carb, you should also spend the money for setting it up using a dynamometer.
    #78
  19. fast4d

    fast4d Long timer

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    I switched to a FCR41 from BST on my duke II.

    lots more torque and response and BETTER MPG. so far I've gone 110 miles on the main tank. on the rejetted BST I would need reserve before 100.

    the FCR is easier to start. I never use the choke. just press the e-start and it starts right away. if it's really cold I'll twist the throttle.
    #79
  20. meat popsicle

    meat popsicle Ignostic

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    madmac,

    Everyone knows that a pumper carb will give you better instantaneous throttle response than a constant velocity carb, but those who say the difference is "night and day" probably have one of the following issues at play:

    1) poorly tuned and/or maintained BST/LC4
    2) lack of throttle/clutch control

    I am not trying to take a dig at you, I for sure have issue #2 and might have some of issue #1 :nod, just trying to be clear for the folks trying to use this thread to decide what carb is best for them. I base my statements on the posts of people such as loisu (who started this thread - did you read Post #1?) in his How to wheelie the LC4 thread and creeper's posts on the subject.

    HERE is one very nice and brief post by a guy who has both a BST-equipped LC4 and a FCR-equipped LC4 (with full Akropovics exhaust). Good points in there for people to chew on as they decide what they want. For me the BST's muted throttle response helps minimize my throttle/clutch control issues so that I keep on truckin', whereas with a FCR-equipped RFS bike I got into all sorts of trouble.

    Sure you can learn to deal with the better throttle response, and probably even kick ass with it, but I have never felt held back by the BST. If you do, and you do not have the issues noted above then perhaps you should upgrade (note that creeper did for a very specific set of reasons). Just make sure you are spending the dough for the right reasons.

    Sorry, but the only way a pumper carb will give you better gas mileage than a constant velocity carb is if the CV carb is poorly tuned, maintained, or operated. Please see creeper's post on the subject. :beer
    #80