A little help for a carb noob in Africa...

Discussion in 'The Garage' started by khw, Dec 6, 2017.

  1. khw

    khw Adventurer

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    Dear friends,

    I am writing this from a rather isolated spot in Western Africa which I'm realizing is not the place to teach myself the intricacies of carburetor work. Or is it...

    I arrived here from the US this past summer with a crated and lovely specimen of a 1993 DR650. I ride it in the bush until I have an issue, wait some weeks for small parts, fix it and get back to riding. It's great. I sold my fuel injected BMW before moving here in favor of something simpler. No regrets. And no need for hand warmers.

    So here's my issue. I've never worked on a carburetor. I knew it would catch me at some point in my life. The bike came with a box of parts which includes a few jets and the original needle for the Mikuni BST-40. I figured great - the original owner might have done some upgrades. So I tore it down to do a cleaning and to make sure that it's in tip-top running shape. I should mention that the air box of the bike has been modified (as most have) and there's an aftermarket exhaust.

    The surprise came when I took the carb apart and found that the aftermarket notched and tapered needle is shorter than stock. Significantly shorter. 56mm vs. 63mm. There is no way - even at the leanest notch - that it reaches the jet. The stock needle does. And its diameter virtually fills the tube that leads to the jet. The aftermarket needle does not. Now I don't know a whole lot about these things and I've been trying to find an answer to this... along with learning everything I can about how to clean and adjust it. But shouldn't that needle seat when the throttle is at 0? Have I been running some hack tuning job thinking that it's just an eccentric older bike? Or is there some logic to having that fuel circuit slightly open at rest... I really do apologize if this a silly question. The bike runs fairly well but it's finicky. And with the heat here, I'd like to be as well-tuned as I can be.

    I'm a little out of my depth and far from anyone who can help in person. So any advice is greatly appreciated. I am able to order up a jetting kit and wait a month for it so if I should stick the stock needle back in for the time being and make the proper adjustments, I certainly have the time and understanding to do so.

    Thanks for reading.

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    #1
  2. VX Rider

    VX Rider Long timer

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    I'm going to guess that getting it (the carburetor) back to stock, or as close as possible will ensure proper running.

    That being said...it's possible that needle doesn't seat on purpose, because the idle circuit on that carb is blocked
    and the previous solution was to put the shorter needle in there to allow the bike to run...

    I'm also guessing from your description the off idle response is pretty poor because of this, uh 'modification'

    Short answer, order a really good kit, something with most if not all the brass pieces in it, then...clean the carb, clean it again, and once more
    and then put your new parts in there.

    If you care to mention where in "west" Africa are you?
    #2
  3. khw

    khw Adventurer

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    I'm currently in Sierra Leone. It's an amazing bit of geography for riding.

    Thanks so much for this. Very helpful. And you're absolutely right - the off-idle response is pretty bad and then it surges. I had read that these bikes like to rev high so I thought that maybe that's just how it runs. Now I'm thinking that it might be a bit extreme. I reckon I'll find out pretty quickly if there's something wrong with the idle circuit if I put the original needle and jet back in. Cross that bridge when I come to it. I'll order a proper jet kit and get to work. This is very helpful. Thank you.
    #3
  4. luckner

    luckner Adventurer

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    Hello!
    The circumstances in which you buy/ride the bike allow to know that in a certain moment it do run propperly, isnd it? And in a moment it starts to run unpropperly, is it so? That triggers to supose that another question may be or is gilty for unpropper run, do you have searched in this direction?
    The carburetors from this high performance bikes arent easy to tune. When unskilly hands make nonsense it si difficult to save. Not mentioning you, but the former owner.
    In theory, it is not necessary that the needle reaches to the jet. It works in the tube that supports the jet, exactly in a shorter tube that shows in the carburetors throat, called "needle jet"
    The main jet handles the full theoretical amount of fuel necessary for the engine operation, but the space between the tube and the unsimilar diameter of the neddle allows the final amount that enters the throat. The different "ring" options created between the diameter of the tube and the variing diameter of the needle is called "toroidal surface", and needs to fit certain values put by the designers of the bike.
    Therefore, it would be wise to return to stock values, and then introduce little modifications to fit the running conditions.
    Obviously, also a contact with DR 650 forums would be usefull, nobody can judge better the bike that its owners!
    I am only guessing in this matter because you dont explain much of the symptoms, but it would be also necessary to dive in other questions like valve gaps, the mentioned alteration of the air box and exhaust, etc. Have you checked that the decompressor is working correctly, if it is automatic? Because in certain cases a wheared decompressor can remain working after its intervention is not still necessary, opening minimaly the exhaust valve and creating a failure possible to be confused with other symptoms.
    Kind regards from Argentina, and good luck!
    LUCKNER
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  5. khw

    khw Adventurer

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    Hello and thank you for your detailed response. In the interest of brevity in the original post, I didn't go into deeper detail which might have been helpful. The truth is that I took a risk on this bike. As one always does with older bikes. I gave it a quick test ride and it was packed up and shipped to Africa. It seemed more or less to be fine. The price was good, it has low miles for its age, it was extremely clean. And in truth, it runs fairly well - but as I've said in my previous post, I'd like to be familiar with the system and have it running the best that it can in these somewhat harsh conditions. It's a bit weird in the 1/8-midrange of the throttle. Which I had read is normal for these bikes. I am simply trying to figure out what I'm dealing with in this modified carburetor. The BST-40 is supposedly very forgiving from what I've read but I'm having a somewhat difficult time in trying to find out if this particular needle arrangement is "normal." Compounding the confusion is the fact that the needle was installed without a washer or nylon spacer. It was just dropped into the slide with only the c-clip. So the basic question that I was hoping to find the answer to is whether or not it's normal to have a significantly shorter needle in a basic tuning setup. It is clear from other cues that this was not done professionally. The screws are a mess, the jet is chewed up by poor use of a flat-blade screwdriver... I am not impressed nor do I trust that the correct needle was used. So I agree that I should return to stock and go from there. You've been very helpful in making it clear that it can be quite complicated and I appreciate it. The decompressor is manual and functions correctly. I installed a new exhaust that is more correct than the DIY chopped pipe that was on the bike. The recommended compensation for the K&N filter and airbox venting is usually a slightly larger idle jet... I will probably order a professional tuning kit and do the whole thing properly. Hopefully it runs better with the stock needle in the meantime. Thanks again for your help.
    #5
  6. shovelstrokeed

    shovelstrokeed Long timer

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    Why not ask the mods to move this to the DR650 thread in Thumpers.
    I could give you lots of guidance to carb tuning here but I've covered it enough times that I won't bother anymore.
    As a noob, you do not know what you don't know and will find yourself floundering around and installing wrong parts and the wrong combinations of parts till you grow frustrated.
    Going back to a stock carb is probably the best thing you can do at this point.
    Once you get the bike so it is idling well and performing well at full throttle, you probably won't be anywhere near optimum but you will then be able to describe your issues better and some free advise might just get you over the hump.
    There are a number of skills required for actual diagnosis of carb issues, a lot more than just reading spark plugs or listening/feeling for backfires and sag at changes in throttle openings. The consensus is to go richer and that, most likely is wrong in many cases. This applies to jet kits as well. Just cause an aftermarket jet has a similar number to the jet that came in the carb does not mean the flow rates will be the same.
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  7. baloneyskin daddy

    baloneyskin daddy bikaholic

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    If it does indeed have an aftermarket exhaust,K+N filter and a modified airbox ,going back to stock jetting is a waste of time.These bikes were jetted lean from the factory to begin with, now free up the exhaust flow capabilities and give it a lot more air intake volume and yes you will need a drastic change in the needle.The pilot is probably a size or 2 bigger and a little bigger on the main also but without the drastic change in the needle you wouldn't find a main jet big enough to work correctly.I'd try fine tuning with what you have.
    #7
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  8. MrBob

    MrBob Knee-jerk liberal

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    In the past I've ordered carb mod kits from Sigma 6 on eBay. When ordering, you were asked to complete a checklist detailing location, type of riding, and modifications.The kits came complete and were easy to install. This might be simple solution for you.
    The DR 650 is a simple and strong bike with a deserved reputation for reliability. I loved mine and frequently took it into remote regions of northern New Mexico.
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  9. khw

    khw Adventurer

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    Thanks for this. Your response prompted me to call ProCycle today (which is a PIA with the internet connectivity here) and they were very helpful as well. He basically told me the same thing - with the exception that the needle should still be fairly close to the same length near the middle of the adjustable range - but that the taper and diameter would significantly different given the modifications. The one that was in mine was still dangling in the emulsion tube at the leanest notch with no spacer or washer installed. He told me that it would run OK but my mileage is probably terrible. It's tough to gauge that here considering the condition of the roads (where they exist). The pilot jet is another story - I removed and cleaned it and recorded the size. Still trying to figure out if it's been changed from stock.

    I've installed the stock jet and needle for a starting reference point and I'll see how it runs compared to the shorty that was in there. I'll probably order a proper high quality jet/needle kit as the modifications certainly require a bit more flow. Just maybe not the gush that it was getting. We'll see how it goes now that the float is properly adjusted and everything is clean. I'd just like to get to a point where it's a little smoother than it was. Be the best bush mechanic you can be...
    #9
  10. khw

    khw Adventurer

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    For the sake of a little clarification and specifically to your comments - I took the carb apart for routine cleaning and to replace the gaskets, o-rings, check the float needle... and found a very short jet needle. I found its length and assembly without washer or spacer to be confusing. I am simply trying to sort out what the past owner did and why. And if this is a normal thing to do when re-jetting and upgrading to an adjustable needle. I posted my question here because I wanted more general answers on carburetor function. A larger knowledge base as this carb is used in many bikes. I've poked around the DR-riders group and read many threads here on carb work so I'm really not out to waste anyone's time. Especially yours. I certainly didn't ask for a treatise on fine tuning. You felt compelled to weigh in and I thank you. Condescension aside, I appreciate it. I'm just trying to sort out the best direction to go in getting this bike running a bit smoother than it was. The other cues that the original owner/modifier left - chewed screw heads etc. led me to believe that this might not have been a particularly meticulous modification. That's why I'm asking if a shorter needle is a common course to take. I appreciate your help.
    #10
  11. VX Rider

    VX Rider Long timer

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    Looks like you're headed in the right direction.

    It takes quite a bit more creatively to get along in Africa than some realize.
    I spent some time in South Africa....and trust there were moments of frustration that were only resolved by reminding myself, that I was in Africa and it is a different place for sure.
    #11
  12. ttpete

    ttpete Rectum Non Bustibus

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  13. shovelstrokeed

    shovelstrokeed Long timer

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    Wasn't meant to be condescending, fact is, carb tuning is a bit of an art and changes made are not at all simple. There are different solutions to the same problem and not being able to hear the bike run and feel what it is doing can lead someone who genuinely wants to help into a maze of garden paths.
    Make sure, when you return the bike to stock condition in terms of the carb that you are sure that the factory slide is part of the equation. The shape and size of the slide cutaway are critical to low/part throttle response and often overlooked in tuning attempts.
    It sounds to me like your carb is equipped with some form of power jet (a brand name). Not the easiest to tune for.
    #13
  14. baloneyskin daddy

    baloneyskin daddy bikaholic

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    I went back and looked at the 2 needles and I bet if you put the thick plastic spacer back under the clip on the stock needle where it s supposed to be the lengths that actually extend into the needle jet are very close.
    #14
  15. JCool

    JCool Long timer

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    The aftermarket needle , in comparison to the stock one , makes me think Dynojet. Dynojet uses a thinner , shorter needle to work with a stock pilot jet and their own larger main jet. The jet sizing numbers are NOT the same , JetsRUs has a crossover chart. If the bike ran good to begin with , then it's not a jetting or needle problem. Randomly changing parts , pretending you know what you're doing is futile.
    #15
  16. khw

    khw Adventurer

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    It's funny - I was just looking at the installation manual for the Dynojet kit when your response popped up - given that it's a common option. The parts and configuration lead me to agree with you. I'll have full confirmation on that if they get back to me on the needle dimensions. I wish they'd stamped the it as it would have eliminated some guess work but everything seems to match and make sense.

    I really do appreciate the helpful part of your answer. But I'm a little confused about the 'random parts' and the 'pretending' bit of your response. Or is it just cultural on this forum for some 'old timers' to drop an insult after a genuinely helpful answer - so as not to seem too welcoming or friendly to those of us who honestly come here to avoid screwing shit up in Africa or wherever. Really appreciate the help. Thanks.
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  17. shovelstrokeed

    shovelstrokeed Long timer

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    KHW, you seem a little tender, we are not insulting you. Please understand, this topic, in one form or another, comes up at least once or twice a month. Over the 15+ years I have been here, that is a lot of answering, to the point where it gets somewhat annoying, especially when, all to often, it is a total lack of accurate description of the problem. Quite often it is about a bike with aftermarket exhaust which may or may not have any effect on low speed running. Typically, these things flow exactly the same air as the stock exhaust and especially at low speed where resonance effects are minimal. The same applies to airbox mods and pod filter type changes. The primary effects are at high engine speeds and maximum flow conditions but it is not unusual to find an engine has gone rich at these speeds due to changes in the reversion where the carb is actually breathing part of the early strokes mixture in addition to what was put in there on the current stroke.
    Trouble shooting these things over the phone or in print is insanely difficult. You gotta be there and listen to/feel the bike to even begin to guess what is going on. Dead stock is the way to start trouble shooting and then knowing the difference between a lean stumble and rich bog can give you clues as to where to go.
    #17
  18. khw

    khw Adventurer

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    Tender, sure.

    I begin to wonder about the purpose of these forums for some folks if it's so tiresome to answer genuine questions. I asked a really specific question that I was having trouble finding a solid answer for by other means. I'm sorry to be one of the monthly sad cases but if you take the time to answer with anything more than the classic 'search the forum before posting,' I just don't get the need for tossing the dismissive negativity as though you've never had a technical issue that stumped you. I honestly thought I was being pretty accurate about the problem with the photo of two needles and "is this a normal thing to find in a Mikuni BST-40?" I do appreciate the more detailed responses on how the larger system works but I certainly didn't post that initial question with the hope of going to free carb school.

    That said, I've learned a great deal from this conversation and have found information that's pointed me in some good directions for researching my issue. I'm specifically not asking any of you to troubleshoot my throttle issues remotely so I feel like the critique is a little off-target. I was not having any luck in searching this forum or others on the normalcy of a short needle. I spoke on the phone with folks who sell tuning parts for this bike and carb and it was also a bit of a mystery. So I posted here and in spite of the feeling from a couple of you that I'm somehow wasting your time or in no position to understand the complexity of what I'm not able to ask in the first place, it was worth it. The information that you again provided is excellent and totally useful. You seem to know what you're talking about and that's why I came here. I'm not sure that I understand it entirely but I'm trying my best with the hope that I can improve my situation and start to do so on solid footing. It's really not my problem that you're weary of 15 years of answering dumb carb questions. It certainly isn't my intention to annoy people by posting here. Believe me I'll weigh it carefully before doing it again.
    #18
  19. shovelstrokeed

    shovelstrokeed Long timer

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    OK, I've advised you twice now to obtain the stock parts for your motorcycle, including the slide and install them based on the factory manual for your bike. Start there. You should be able to get the bike to idle and run strong through the mid range using the stock parts. I know, Africa.
    A plug chop, look it up, will handle the upper part of the rev range.
    Here is where you can run into some trouble, again, with a verbal description of the problem. What is low range to you? Where, in terms of RPM and gear do you run into your issue with the carb? Are you sure it is carb related and not ignition related or just crap quality gas? We are talking about a 24 year old bike here, how is the compression, what about valve adjustment, are there any leaks in either the exhaust or intake? Is the ignition timing set to factory specs and is the advance system working?
    I've been wrenching on motorcycles since around 1960 or so and cars before that. My experience tells me that unless all of the things mentioned above are in specification, fooling with carbs is a wasted effort.
    With answers to the above questions, I can, perhaps, provide you with some more detailed help or, at least, guidance.
    #19
  20. khw

    khw Adventurer

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    Now this is excellent. Truly. I can't do anything about the crap gas here but I can certainly work through this list and will enjoy doing it. Believe me, I've heard your advice on the stock parts. I have the jet and needle, I'll order a slide just to be sure. Government mail is slow but gives something to look forward to. I have the shop manual and factory specs for everything so I'll set to work with the other pieces of the puzzle. Thank you for this and for your time.
    #20