Which carb is best on LC4 640

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

  1. Hipster

    Hipster Long timer

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    Losie,

    The TM40 carb has 3 different throttle return spring settings, from light similar to the BST40 to very firm. This may have been what you were experiencing with the bike you road.

    Tomc
    #41
  2. mike cramb

    mike cramb Long timer

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    OK meat Sir Popsicle that is i will try the standard bst carb with secondry air systen blocked first. Might save myself A grand the guy who just fitted the keihin had that mystery problem of breakindg down,missing when riding over jumps , or pulling neg G "s so his will be better mine doesnt have that problem.

    MIKE:D

    See if you can help with my brakes!!!!!!!!
    #42
  3. meat popsicle

    meat popsicle Ignostic

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    You need the front reservoir off of the 2003 or earlier 640a eh? I'll take a look in the threads when I have the chance. Many looked for a different part since the KTM OEM stuff is, argh, pricey.
    #43
  4. losiu

    losiu wheelie addict

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    Thanks for that! I knew there was something screwy about it :D
    I actually bought a keihin CVK from a KLR to trade for the TM40 - I had it all arranged but then the bike got stolen and now I'm stuck with no bike and a CVK in my garage. I might try to get that TM40 for the future cuz the guy doesn't even ride the KLR and basically doesn't have a problem with switching back to CVK.
    I had no time to mess with the TM40 but I believe it really IS a cheaper alternative considering the price of an FCR.
    Once again - thanks for the hint :)

    Losiu
    #44
  5. losiu

    losiu wheelie addict

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    Time to jump-start this thread.

    I have a question to those who have actually ridden an LC4 640 with different carbs on it.

    Since I started this thread, I've learnt a lot about the carbs on LC4 (thanks to all the replies here). I have a serious question concerning carbs, though.

    I test-rode that LC4 on steroids I mentioned a few posts back and it had a MIKUNI TM40 carb on it. The bike had the following mods:

    1. 660 cylinder
    2. 660 rally cam shaft
    3. Double valve springs
    4. TM40 pumper carb
    5. Full titanium AKRO
    6. Polished head ports

    It was making 66.6HP on the clutch (supposedly tested)
    When I took it for a spin, it felt incredibly smooth and almost no vibes.
    But I have to admit that while it was crazy fast up high, it lacked the brutal throttle response I liked so much on my almost stock LC4 with cheap, popular mods.

    I already know the brutal response is caused by the roughness of the BST carb. By roughness I mean that the engine vibrates a little more and doesn't feel so smooth at lov revs. Everybody will agree that the hick-up of the BST in the whoops is a really bad thing but what about the overall performance and power output of the BST compared to other carbs discussed in this thread (FCR, TM40, Dellorto - mentioned a couple of times here...). Is it really that bad?

    I'm asking because that 66,6HP KTM didn't impress me with ANYTHING at all. If I wanted a bike that was fast on two wheels, I'd look for a GSXR or R1. I'm looking for a thumper with a thumper-like power. I want my bike to have a proper throttle response at any revs.

    After riding a stock LC4 640 and one with a TM40 installed, and one with BST+exhaust and airbox mods and the 66.6HP KTM as well, I'm a little confused.
    I'm not so sure that the mighty FCR41 would be good for me and here's why I think so:

    In Europe, most car dealers offer cars of relatively small displacement and attract people with the HP of the engines.
    I personally think it's a big joke. I've had a 116HP, 2.0 MAZDA 626 which was as fast as the same MAZDA with a 136HP engine (except for the top speed).
    Right now I drive a 2.0 Turbo Diesel station wagon that makes 130HP and has twice as much torque as the Mazda at only 1800rpms (instead of almost 4000rpms of the MAZDA). This car accelerates as well as a 150HP gasoline 2.0 of almost any car manufacturer you can name.

    What I mean by the above example is that really high power almost at the red line is not worth a thing. How often do you rev your bike so high that the limmiter has to do it's job? If you race your bike, then probably every day. I ride my bike every day to work and need power throughout the whole rpm range, not only at the top.

    So is it me or is a modified BST actually pretty good considering overall power at any revs? Do you have any suggestions for a guy that is looking for tons of torque and low to mid-range power???

    I'd appreciate any comments.

    Cheers,
    Losiu

    P.S. Sorry for using cars as an example. I just think that they show my point best. A 2.0 with 165HP at 6000rpms is a joke. How often do you see 6000rpms in your car??? I feel the same about the power of my bike. It just needs to be lower so I can use it.
    #45
  6. enduro-ince

    enduro-ince dirtslave

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    Holy Crap! This is quite the carb thread. I couldn't do it. I couldn't read all of it. I know, its only three pages long but man thats alot of info. So I'll jump in with some seriously stupid questions.

    What comes stock? I'm pretty sure thats what is in my bike.
    I've been very pleased with the performance of my 02 640 a. But I wouldn't be surprised at all if I could improve the "Thump" in my thumper.

    What Carb would be best for higher elevations? 7-12000 feet?

    Currently my bike has some trouble above 12500 feet.

    Am I totally missing something by running the stock carb? Wheelies aside, I wouldn't mind some more grunt for getting that front wheel up in a hurry.
    #46
  7. humdinger

    humdinger bp

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    Clocked just over 5000kms on my '05 Adventure and after researching and reading feedback (alot here on ADv.Rider) have decided to go for the FCR carb in place of the modified BST.
    I'm no 'gun' rider but I like a very responsive throttle action when riding either off or on road and after much testing and riding of similar machines with both stock and highly modified engine componentry, to me, I like the versatility and overall performance characteristics of the FCR, especially when combined with a more free flowing intake and exhaust system. Yeah, I love tinkering with the beast too but its nice to have a set up that feels like your throttle is connected directly to the ground when opened up under all conditions and that is what I'm hoping I will acheive.
    I'll 'echo' what has been mentioned here by losiu in stating that more power doesn't necessarily mean you will accelerate more quickly or give you a more responsive feel; only fine tuning will acheive that with a compatible combination of whatever componentry you choose.
    My first step will be the FCR carb and then I'll be looking at a full exhaust system that breathes well, is much lighter than stock and is reasonably priced. I'm currently using a hand made air filter that has a much greater surface area than the original and have noticed a marginal gain in engine performance but doesn't clog full of dust as quickly. I'm confident that after making the above mentioned changes (carb and exhaust) that engine performance will be more noticeable.
    Thanks to all that contribute to these threads and I will add more feedback as I complete my modifications.Have a nice ride people
    :D
    #47
  8. lvdukerider

    lvdukerider Las Vegas Nevada

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    I have had the FCR 41 on my 00 Duke II for several years now and that was after doing the mods on the BST 40. There is nothing wrong with a well set up BST but IMHO the FCR 41 is the carb all LC4's should have come with from the start. I installed mine on a whim just because my dealer happened to have the Sudco kit on the shelf and I wanted to see what all the hype was about, I only wish I had done it sooner. I think the lack of low end on the bike you rode might be due to a high end cam, tuned for top end H.P. at high R.P.M at the expence of low end torque. All I can say for certain is that my Duke II has much more low end snap with the FCR 41 than it had with the modified BST 40. Oh and if mileage is a issue the FCR 41 does use more gas than the FCR 41.
    #48
  9. losiu

    losiu wheelie addict

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    Sorry for my absence. Had to buy myself a bike in the mean time...:D :D :D

    Well, sounds like the FCR is still the winner. Probably should be because it costs so much in Poland that with a bit of luck you could just as well buy yourself a cheap bike to ride...

    humdinger, as soon as you figure out something new that works, please post it here. I'm really curious...

    enduro-ince, find any of Meat Popsicle's posts and read the LC4 index thread. There's a lot to learn from there. Or simply read this thread from the beginning - the answers are here :)

    I'm glad you've posted guys. All I know about carbs comes from this forum.
    Just bought a 2001 LC4 640 and I have the LeoVince silencer and no pre-silencer. Sounds way cool but the BST is really worn out (slide, needle) so I'm installing another BST (can't afford anything else right now) and I'm going to re-jet it as usual. We'll see what happens :)

    Losiu
    #49
  10. planetmike

    planetmike Been here awhile

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    Anyone have an active link to the instructions for installing this carb in the Adventure (with photos)? Also, I actually have the HS40. So any comments on slight installation variations from the TM40 would be most helpful.

    Thanks!
    #50
  11. trond

    trond Adventurer

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    My LC4E came with the BST. I overhauled and drilled and got the Schneiders tuning kit and it was smooth and strong but not raw. So I got a Dell'orto 40mm and spent time and money setting it up and the bike was more spontanous, engaging and fun but I don't believe it had more power. I wheelied better with the BST. I then put on a FCR39 and spent a small fortune setting it up. I even got a A/F sensor and it is a great carb. It needed much more work but even slightly out of tune the throttle response and fun factor makes it the best carb yet. Then last weekend the thing started leaking from the overflow tube and I could not find out why, so to keep me going I temporarily put on a FCR37mm from a Yam250 and guess what, IT WORKS GREAT ! The midrange is humongous, better than with the 39. And I'll be damned if I can notice any loss of power on the top (admitted I only went around the block at night wo helmet and with flip flops). But for offroad/enduro use it has enough power and I will not put back on the 39 for a while. I will go to the desert in the weekend for some open desert testing and maybe try changing clip position. Current set up is 155 m/j, 42 i/j, OBEKT 4th pos, 50 leak jet and long stroke diaphragm in the acc pump

    Oh by the way the acc pump hits the exhaust header pipe which will have to be dented quite a bit
    #51
  12. losiu

    losiu wheelie addict

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    Hey trond, amazing stuff you're saying !!!
    Go test the bike and post your thoughts here.

    I heard that the acc pump cover might even crack from overheating and being in contact with the headers. I don't know what solutions there are to avoid it. Maybe a 2003 header which is shaped in a way that doesn't colide with the FCR... ???

    I'm dying to hear how the bike runs after you take it for a proper ride !!!
    Don't even think about leaving this thread :lol3

    Losiu
    #52
  13. trond

    trond Adventurer

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    Yes, you are right, the acc pumpcover is an issue. There are different versions, I have the one that looks like a jellyfish, with hoses all over and one that looks like the pic in the link below, with a box on the left side and TPS. I believe they are both the MX version, with float chamber baffling against vibration. Anyway, the acc cover hits the header pipe on both. I dented the pipe with a hammer and had some 4-6 mm clearance but still it hits because of vibration. So I took the trusty angle grinder and took off some material and still it rubbed occasionally until it finally wore through and started leaking. So you need to ding that header pipe proper to get some clearance. Not to the point where it matters for the exhaust but I believe you need more then 5 mm clearance. Also I must warn against the wider version from the Yam450YZF, it is too wide to fit between the frame tubes without heavyhanded use of the angle grinder and then sealing up with epoxy afterwards. Not for the faint hearted. Here is a picture of the version that will not fit
    http://cgi.ebay.de/Keihin-FCR-40-Ve...ryZ86749QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
    #53
  14. losiu

    losiu wheelie addict

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    Are you sure that what you did didn't affect the performance ? I mean "denting" the header to make the carb fit... ???
    I'm not the kind of guy that'd use a grinder either... :eek1

    I am really curious of your comments after a longer ride with that carb on. The weather is crap here so no chance to ride my wheelie beast :(

    Losiu
    #54
  15. trond

    trond Adventurer

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    OK I am out of hospital after the weekends ride ! Hit a sharp bump and went over the handlebars. Nothing broken, ass double size and sore all over. Bike is fine except bent rear subframe. Really tough bikes.

    On the road it does 150 km/h easy, with more power available but this is about my limit with the Michelin Deserts. I never used full throttle yet, which indicates the 37 mm is big enough for me. And still the midrange is really excellent. I lowered the OBEKT needle to mid position and moved from 50 to 90 leak jet to reduce fuel from acc pump with no apparent loss of power. Slightly rough at about 70km/h in 5th so will try different idle jet because I believe it might be mixture related. Not a problem offroad in the sand where the engine is loaded a bit more. For enduro with the pre-hiflow engines the FCR37 is on top of my recommended list. Also easy available as a lot of 250 crossers use it

    Btw the angle grinder is NOT required except with the wide body YZF version. Denting header pipe is required, at least with the small header pipe and pre-hi flow pipe. Don't worry about power loss, the dented pipe with the FCR is miles better than anything else
    #55
  16. losiu

    losiu wheelie addict

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    Trond, the stuff you write amazes me every time.
    I hope you feel good anough to ride and keep posting your thoughts on the FCR37mm carb. I don't recall anyone ever putting this kind of carb on an LC4.
    Very interesting... :eek1

    Losiu
    #56
  17. trond

    trond Adventurer

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    Interestingly the FCR37 measures closer to 38 mm ! Now, the LC4s used to come with Dell'orto 38mm carbs. So it is not too controversial at all. Most people don't know, and most people, myself included, have a tendency to believe bigger is better, like bigger jets, bigger carbs, wilder cams, louder exhaust and so on. Not always the case. My bike is faster, both acc and top end than my friends big bore, rallye cam, rallye exhaust Adventure. It is also very much less noisy. That was when we both had the 39mm carb, haven't tested after I switched to 37. The only difference was I spent some time to get the jetting right and he did not. There can be a surprising power benefit to be found making what you have work at it's optimum ( I still recommend getting a FCR (39 or 37) but you have to spend some time and make it work properly ! )
    #57
  18. Kaineb

    Kaineb Mud Magnet

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    Hi,

    Love the thread. Can anyone comment on what a starting point would be for a 00' lc4 400 ? Comes stock with a bst and the stock jetting on the bst is 142.5/45. So please understand the stock bike is lean.

    Also am going to buy the ktm performance quiet muffler to replace the stock. I bought a side screen but not sure i need it with the fcr.
    #58
  19. trond

    trond Adventurer

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    don't know anything about the 400 but that dosn't stop me ! I would guess a FCR35 would be a good choice, or a 37. It is generally easier to set up a smaller carburettor. Both can be found at low prices on eBay. Don't know about setup but I would start with 45 idle jet, 150 main jet (impossible to tell so a wild guess), 100/200 air jets (standard) and OBEKR needle. Mind you these are wild guesses with no backing. Comparing venturi area divided on capacity for 400 cc and 625 cc tells you a 32 mm carb on a 400cc is similar to a 40 mm on a 625cc. Obviously this is an oversimplification as the 400 revs higher, but you get the idea.....
    #59
  20. losiu

    losiu wheelie addict

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    Maybe it's time for me to start looking for a smaller FCR. I think I'd be happy with the 41mm but I like mid range power and I think a 39 (or even the 37 you're describing) would be better for me... interesting...
    #60