Which carb is best on LC4 640

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

  1. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,643
    Location:
    Circumlocution Office of Little Dorrit
    WHOOP! there it is!

    You know, ahem, I've been wondering (my mind wanders into all sorts of trouble, :lol2 but nonetheless!) perhaps the appeal of the FCR and its ilk :pope is the relative ease of extracting performance in comparsion to the BST. :evil Note that extracting was carefully chosen word.

    Perhaps it is easier for those who are less skilled at tuning to extract more performance from a FCR, which has more inate ability in this regard, than a BST, which may require more fine-tuning for equivalent perfomance. Put simply: a hack tuning an FCR is going to get more throttle-smack than a hack tuning a BST.

    (cut to the chase) So the question remains how much better is an FCR than a BST? And for those who care, is it $500+ mo'betta? I propose this: someone send their kitted Keihin FCR to creeper (in good shape mind you) so that he can set it up to the best of his ability :wink: and compare it to his own BST on his very own bike. That should allow for a valid comparison IMhO and provide this discussion with the eulogy it never deserved. :deal

    Oh yeah, I believe he would like a FCR39... I say this because he just might do the comparison if he gets to keep the carb. :lol3

    PS - I too would probably be able to extract more performance from a FCR than a BST, so don't think I am feeling all high and mighty. :smooch
    #21
  2. Hipster

    Hipster Long timer

    Joined:
    Aug 31, 2004
    Oddometer:
    2,572
    Location:
    Mpls, Mn.
    I have the Mikuni TM40 installed on my bike and for the $250 investment it was well worth it.

    I set mine up per Flanny's instructions and the bike runs better then with the stock BST40. It starts easier, and throttle responsive is much better from the low to mid rpm range.

    Tom
    #22
  3. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    I've been thinking....
    There have been lots of carb threads here and as I mentioned before, some posts surprise me. Let me explain:

    Flanny said he can now (with the TM40) wheelie in 3rd gear using small hills or bumps on the road (don't quite remember how he phrased it). I love playing around on the bike and totally enjoy the idea of having more power than I need just in case I need it at some point. I'm not a fan of speed - power is what I like. I learned to wheelie on my 2001 LC4 - completely stock. Until I knew how to do it properly, I thought it was barely possible in 2nd and totally impossible in 3rd. Later it turned out it was totally possible in 3rd and super easy in 2nd. Now I have the 2002 SixDays with all the popular changes. The bike's performance is much better. I wheelie in 3rd without any problems as long as the carb is clean. I never used the 1st gear to start a wheelie unless it's my show off one when I start from stopped and go through all gears on one wheel. All of this was always done with the BST on. I managed to do a wheelie using a hill on the road in the 4th gear on the stock LC4 and did the same on some kind of bump on the rejetted BST in my 2002.

    As I mentioned in one of my earlier posts, I need the bike to be reliable so exhaust, intake, carb mods are the only one I'm considering. Now let's get back to the carb issue...

    I'm slowly starting to believe in one's ease/ability to extract power out of an LC4 (btw, extracting is a pretty good word here). A pumper carb will definitely help but if a BST has that much potential (haven't even done any drilling in mine) than it's a good question whether spending hundreds of $ will pay off... Based on my test ride with the TM40 I think there is a slight improvement in acceleration, especially from mid to high rpms. I might be wrong cuz it's not properly set up yet but....

    I'm under the impression that many people here compare the STOCK BST to pumper carbs which actually are similar in performance to properly set up, rejetted BST with screened airbox and a muffler. I'll try to work on the TM40 to get a decent set up and maybe I'll change my mind.

    It's getting longish so I'll try to sum it up.
    A properly set up, rejetted BST provides pretty good performance. NOT A STOCK ONE ! Out of the available pumper carbs the TM sounds better because it's cheaper than an FCR. I don't know if the FCR is actually worth the money unless someone uses it for racing where it's really useful.

    I don't want to offend any TM40 or FCR owners. I still like the idea of getting a little more snap and performance out of my bike through a pumper carb. What I wrote here is purely theoretical (I'm a thinker) and I'd be more than happy if someone proved me wrong because I want to believe that the TM is much better than the BST.

    Thanks for your patience :)

    Losiu
    #23
  4. Hipster

    Hipster Long timer

    Joined:
    Aug 31, 2004
    Oddometer:
    2,572
    Location:
    Mpls, Mn.
    Losiu,

    I was compairing a re-jet BST carb to the TM40. The main reason I upgraded the carb is do to the BST's inability to hold a steady rpm while riding through deep whoops.
    #24
  5. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    I noticeda few issues with the BST. One of them is thee common thing in the whoops - a kind of hick-up thing. Another thing is the roughness at low revs - especially when it gets dirty around the vacuum piston. It also loses power whan that happens (a little but enough to notice).

    I did some more riding with the TM40 and the most amazing thing is its smoothness at low rpms. I have to admit that it's much nicer to ride an LC4 with a carb like that for THAT reason. No vibrations and smooth power delivery. Unfortunately I couldn't experiment too much with the carb because it wasn't mine. I put it back in the KLX I borrowed it from.

    I didn't notice the snappy throttle response but I think the squirt wasn't adjusted properly for an LC4. Not to mention the jetting. In the mean time I did the drilling and spring clipping in by BST and put it back on.

    The bike has better throttle response than before. Overall performance is comparable with the TM40 I had on but I'm slowly starting to get what it's all about. BST sucks at low revs....

    Los
    #25
  6. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,643
    Location:
    Circumlocution Office of Little Dorrit
    Thank you loisu for much more eloquently posting what I had tried to post (perhaps my slapdash attitude on this subject does not come across well...), including personal experience of the problems that some BST users have complained about - and what reads like some insight into their aetiology, which I am very interested in. It is this experience that I would like to trouble you further. Here are some quotes for example:

    I do not recall a post here on maintaining the BST, that is unfortunate eh? I am about to rebuild mine, which is at 10,000 miles (did I wait too long? the fuel efficiency and leaky overflow hose leads me to believe I have...). I have not once opened up my BST. (Should I admit that?) How have you maintained your BST, and how often?

    Perhaps many who experience problems simply have dirty carbs. Again, your experiences here, and your maintenance schedule and routines would be of assistance to many. Do you have time to compile a How to Maintain Your BST thread for us? :deal

    Speaking of contrasts, I had the opportunity to ride a fella's RFS bike today offroad. Throttle response was VERY different from by BST-equipped LC4 (which is in need of maintenance and not setup by a pro). I got to feel the SNAP that folks are talking about! It was startling, and very exciting as well. But the experience drove home a bit of technical info posted by creeper (who is very knowledgable about carbs), namely that the BST's smooth power delivery should actually help someone with less throttle control (or clutch for that matter) in offroad riding. I realize that you are focused upon street riding, and smooth does not help with wheelies (sorry I might have contributed to your forgetting to wheelie :cry), but I thought you might have some comment about this "feature".

    As a side note, do you think that a soft rear suspension can have an influence on instantaneous throttle response? I know there are lockouts for forks, so that racers can get off the starting line quicker, so I thought perhaps the rear can be a factor as well. I followed a fella on his LC4, that had the suspension professionally setup, who was bunny-hopping off little bumps, which my bike just soaked up like a Cadillac - got me thinking. Yes, I too am a thinker, who probably needs to ride more!

    If you get the time and inclination, some folks might appreciate another how-to from ya: How to Wheelie Your LC4. Could be a fun thread. :deal

    BTW, I also notice the lower vibe comment with the TM40 - very interesting - I hope you get one so you can finish your experimenting.
    #26
  7. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    OK... here it is:
    I'm not sure I have enough knowlege to write about carbs. I wouldn't have posted the questions starting this thread if I knew much about them. Feel free to use what I write here to start a new thread. I have noticed a few interesting things with the BST. Since I do wheelies regularly, I noticed that sometimes the bike loses power when the front wheel is up. Wheelies are then impossible to control and instead of looking cool, it looks like I'm struggling to stay in charge...(and that's not good when you're riding on one wheel). When that happens, I know that it's time to look inside the carb. I hate taking the carb off the bike, even thoough it's quite easy and with a little practise doesn't take long at all, so I usually just unscrew the 2 screws on top of the carb to get to the vacuum cylinder and take it out with my fingers. That alone will in some miraculous way improve the performance of the bike (after you put everything back in the carb, of course). But since it's out, clean all the dirt that's in there. Usually there's enough to see it clearly. After that's all clean, put everything back together and go riding.
    The bike will run smoother, in most cases it will eliminate some of the popping on decceleration and...
    When there's dirt/dust in there, sometimes when riding at very low steady revs (e.g. in traffic in 2nd gear) I feel that the power delivery is not smooth and there are "fluctuations" in the way power is delivered. If I'm not moving the throttle at all, the revs should be steady and I shouldn't feel any roughness. Unfortunately that's also one of the hints that it might be time to take a look in there. I've had a 640 for 5 years and it does happen from time to time. Use whatever you want to clean the inside of the carb. Q-tips work fine.
    I'd recommend cleaning tha area of the carb at every check up (5000km / arnd 3000miles).

    Smooth power delivery of the BST helps in a sense with wheelies. It may be easier to do a wheelie having a pumper carb because of the better throttle response but after drilling the holes in the vacuum cylinder I can't complain. So if you have decent throttle response to start a wheelie, it's easier to continue it having smooth power delivery. If it's too snappy, it might be more difficult to control. On the other hand, everything is only a matter of time and practice. There are people who supposedly wheelie GoldWings !!!


    Well, I guess I could start a wheelie thread with some advice for those who'd like to actually learn, not destroy their bikes and health... I've read the forum from time to time but can't be sure whether threads like this have appeared here (and how many of them) so I was going to be careful with what I post :)
    If someone with as many posts as you meat popsicle thinks it'd be useful and fun, I'll get to it... :wink:

    Back to the carb... the smoothness of power delivery of the TM40 I had on my bike was amazing. I have no idea if it was properly adjusted or not. I had no time to mess with it enough. I don't know if every TM40 delivers power the same way, but it seemed you couldn't rev it too low. Normally in 5th gear th lowest speed that the LC4 engine accepts if you want to accelerate is around 80kph. On TM40 I could get as low as 70kph and I didn't hear any strange noise or feel the usual vibrations when speeding up. I can't say the engine was super powerful down there. It was just.... different and kinda cool. Definitely much, much better than BST. I guess that if you consider that it also improves the performance a little and properly adjusted adds more snap, then it's definitely a nice thing to have and even nicer to use.

    I imagine that throttle response doesn't change with a stiffer shock. The grip might change but only in the dirt or loose stuff. As you already know, I don't go offroading that much. I simply have no time. I even have another set of wheels all ready to go... :cry

    One more thing about the BST I just remembered... Imagine you're doing a stoppie with the BST. When the rearr wheel goes up, the engine stalls and there is a little puddle of gas under the bike. The hose that the gas spills through is the same one used to drain the carb - at the very bottom of it. I don't have this problem because Instead of letting it go straight down and through the zip-ty by the swingarm axle, I put it up and around the main gas line (the one from the tank to the carb) and then down where it normally goes (it's long enough to reach). There you go - no gas spills or engine stalling :clap

    Cheers,
    Losiu
    #27
  8. Lornce

    Lornce Lost In Place

    Joined:
    Aug 17, 2003
    Oddometer:
    23,067
    Location:
    Way Out There.
    Fascinating thread. Good on you Losiu and thanks for your contributions.

    In an effort to preserve economy as well as improve perfomance I've spent a bit of time modding cv carbs to work/perform more like slide carb on a variety of bikes... and with some good results.

    Losiu, if you ever come across a stock Buell/Kehin 40mm cv carb (they're cheap as borscht in North America because everyone's replaced them with HSR40's) and you're enclined to experiment, you may be surprised how well it can be made to work. A unique/useful feature of the Kehin 40mm cv carb is it's accelerator pump.

    Thanks again and all the best to you.
    Lornce
    #28
  9. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,643
    Location:
    Circumlocution Office of Little Dorrit
    Perfect losiu,

    Exactly what I was looking for - operational signs of a dirty carb. Sharing your experience with others, regardless of how much knowledge you have of carbs, is priceless. You have qualified your statements very well too! I would go open mine up right now, but I have to be up for work in 6 hours... so I am off to bed shortly. This weekend will have to do.

    There are a few reliable sources of info here that like to wheelie, dirtrider for example. Then there will be some steroid junkies, no biggie. The biggest hit will come from the Peanut Gallery, but you just have to learn to enjoy them! I think it could be fun, especially with you teaching folks how to wheelie their BST-equipped LC4 :evil

    Again, I hope you can get ahold of a TM40 for your experiments.

    I note the TM40 listed by Sudco has an accelerator pump. Maybe you said that, but it's late! Good night!

    I was thinking that a softer rear would absorb some of the energy being transfered from the bike to the ground instead of it being translated into forward momentum. Perhaps this is more important to wheelies... but I am just thinking out loud on this one...

    Nice mod tip Losiu, I will have to take a look at this one and my slide ASAP!

    Cheers,
    Meat
    #29
  10. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    Thank you Lornce,
    I'm glad I could help you guys with my BST experience.BTW The little trick with putting the "drain hose" of the carb up and then down has no side effects. It ONLY makes things better :lol3

    Lornce!! Does that Buell/Keihin carb have a symbol or something I could use to find it?? Where I live Buells aren't that popular. And knowledge about carbs, tunning them or switching to something else is generally limited. I'm lucky I know English in that respect. I've learnt a lot from ADV Rider and reading other forums :)

    I found a used TM40 for sale somewhere else in Europe but the price is 150Euro. Not the best deal...
    If I could get the Buell carb, I'd give it a try. Can you write more about the performance and what one needs to put it in an LC4 ???

    Losiu
    #30
  11. Happe

    Happe Adventure Bike Spanner Man

    Joined:
    Sep 25, 2003
    Oddometer:
    694
    Location:
    Wales UK
    Hi mate,

    your description looks like a worn out slide. and a not propperly set float level.
    Might be time for a general refurb or a new carb.

    Sommer in Germany sys you should change the slide in an BST every 20000Km including needle and needlejet.

    And if you find that much dirt in there you should put some mfine mesch over all the holes and air lines.


    cheers
    Stefan
    #31
  12. tallbob

    tallbob Been here awhile Supporter

    Joined:
    May 23, 2005
    Oddometer:
    811
    Location:
    Butt Mt. VA
    Lornce!! Does that Buell/Keihin carb have a symbol or something I could use to find it?? Where I live Buells aren't that popular. And knowledge about carbs, tunning them or switching to something else is generally limited. I'm lucky I know English in that respect. I've learnt a lot from ADV Rider and reading other forums :)


    They are on all carburated Harleys from 1990 or so, not just Buells. I believe it's the same carb that's on KLR650s. You still get a little CV throttle lag but they do have an accelerator pump which may them a little better than the BST. The Keihin has a heavier metal slide but Dynojet sells a plastic slide. Sounds interesting, if I had some extra time someday I might try the swap since I have both bikes. Oh well I gotta go to work instead.
    #32
  13. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    Sorry Happe but I disagre here. I may not be the most experienced LC4 user here but I've noticed that that after some time the bike's performance gets a bit worse (just a bit). After I pull out the slide and clean it + clean little bits of stuff inside the carb (ot the top surface - that is probably irrelevant but I do clean it since the slide is out) the bike's performance is a little better. It's my 2nd LC4 and they both react in exactly the same way to cleaning the slide area. The thing is that the one I have now I bought with 1500km on it and it looked brand new. The previous one had over 40.000km on it when I sold it.
    Based on my experience the needle needs to be replaced because it gets worn out unevenly from vibrations. My buddy's bike was acting weird because the needle was worn out. It was probably around 50.000km the slide was never a problem. I currently have 12.500km on my LC4 and it's simply impossible that anything is worn out, especially that I do mostly street riding.

    I do believe that the dirt gets there because of all those little funny filters around the carb. After making sure that all that crap doesn't even get inside the carb there should be no problem. I'd still suggest cleaning the slide at each check up. It's only 2 screws and It usually made a slight improvement.

    Of course I'm not saying that worn out slide doesn't act the same. I just know that's not the case here :)

    Tallbob, thanks for advice. I'll keep my eyes open for a carb like that :)

    Losiu
    #33
  14. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    Found somewhere on thumpertalk that drilling the slide helps again in the KLR carb. I guess it's a typical thing for CV carbs...

    Losiu :scratch :scratch
    #34
  15. mortaru

    mortaru n00b

    Joined:
    Sep 21, 2006
    Oddometer:
    2
    hello , I am new here but been reading the forum for a while.I owm an LC4 enduro EGS with a 96 engine on it and after fitting it with an adventure tank I need a vacuum source.Do you thiinck that by just drilling a hole into the carb connection would work?
    #35
  16. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,643
    Location:
    Circumlocution Office of Little Dorrit
    See Post #19 in THIS thread for a picture of how it is done in one case.
    #36
  17. mike cramb

    mike cramb Long timer

    Joined:
    Mar 13, 2006
    Oddometer:
    1,312
    Location:
    Perth
    The best carb they tell me and I have been doing some research is the Keihin 41mm pumper. Im fitting one in a couple of weeks when I get back from work. They cost $1095.00 from Show & Go motorcycles in Adelaide Australia as a kit with everything to fit a 640 Adventure .

    I believe its the same carb as on the 625 exc. A mate has just fitted the same and reckons its great heaps more power but fuel consumption has gone up a bit.

    I am also told that if you clean up the ports in and out as well as the above carby they will pull to redline with real results and fuel economy will also improve.

    MIKE:D
    #37
  18. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    Well, actually I'm slowly looking for a replacement bike for my recently stolen LC4. I'm sure it's going to be an LC4 or something very close (SXC).

    'm consideing an SXC 625 because I'd get the FCR41 with the bike and after putting an 18l tak and more comfortable seat on it i reckon it could turn into a pretty cool bike. That's just the theory, though.

    There's also a 65HP KTM for sale now (but winter is coming so it's kinda pointless for me to buy it now) and it already has a ful AKRO, 2 tanks (18, 12) and comfort seat + 660 top end. This bike is fitted with the TM40.

    The thing about TM40 is that I liked the performance (tried it on my bike for a day) but didn't like the way it felt when opening the throttle. It wasn't new and maybe needed some work (cleaning and stuff) but as far as I remember an FCR39 on my buddy's EXC450 felt more like the stok BST (on the throttle- the way it "operates"). I can imagine that on an LC4 the FCR kicks ass.

    Losiu
    #38
  19. meat popsicle

    meat popsicle Ignostic

    Joined:
    Feb 9, 2004
    Oddometer:
    14,643
    Location:
    Circumlocution Office of Little Dorrit
    I can't bother finding the links right now, but didn't someone post dynos? The FCR, with free-flowing exhaust, will net a few ponies. The biggest difference posted by folks who stray from the light :pope is improved throttle response. If you feel that your riding has been hurt by the BST's throttle response, then by all means get a pumper carb.

    Perhaps the "heaps more power" is the difference between an old worn out (unmaintained) BST and a new FCR? :dunno

    The only true "heaps more power" I have heard of is what loisu mentioned: a big bore kit with top end to match, performance cam, free flowing exhaust, and pumper carb. Folks say the LC4 can attain 80+ HP if you squeeze it as much as an RFS bike - just get ready for the maintenance to suit.
    #39
  20. losiu

    losiu wheelie addict

    Joined:
    Jan 7, 2004
    Oddometer:
    580
    Location:
    POLAND
    I have that dyno chart thread bookmarked somewhere but I have a brand new Hard Drive with a brand new system installed so I'd have to switch to the old one to post it. Anyway... I saw what it was about and (this'll make you happy meat) BST turns out to be pretty good surprisingly... the throttle response will always be behind. In an FCR the cable pulls the slide to it kinda goes without saying that the response is immediate :)

    I realize that maintanance might become a problem when you have 660 toys in your engine but I can't imagine it being that bad. I have 2 kids and 2 jobs so I can't really ride that much - I wouldn't even notice (except for te extra performance) :)

    Losiu
    #40