Help with carb jetting W650

Discussion in 'The Garage' started by phq2m8, Jul 1, 2017.

  1. phq2m8

    phq2m8 Adventurer

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    Hi all,
    I bought k&n filters and a carb jet kit for my 2001 w650 from germany and it has no instructions as to how many jet needle spacers are needed.
    It came with 10 of them in the kit.
    im using 38 pilot and 140 main jets, any help greatly appreciated.
    #1
  2. JCool

    JCool Long timer

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    I know on Dynojet's site you can find a pdf of instructions. What jet kit ?
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  3. phq2m8

    phq2m8 Adventurer

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    #3
  4. phq2m8

    phq2m8 Adventurer

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    not a dynojet kit as it has no needles included.
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  5. shovelstrokeed

    shovelstrokeed Long timer

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    Put your stock jets back in and call it a day. The K&N filters won't make a gnat's ass worth of difference in carb air flow. Now, take your bike out and do a proper plug chop and see what that tells you about main jet size. Note: You will probably need a mile or so at full throttle and near max RPM to get any meaningful color on the plugs.
    Can you tell the difference between the need for a richer needle or a richer slide cutaway? If not, why are you messing with carbs that were set up by a very competent engineering department.
    Go through your carbs and make sure all jets are clean, floats set to factory height with no leaking needles and seats and make dead certain that the boots that mount the carbs to the heads have no leaks. While you are at it, put the factory airbox back on the bike and enjoy the smooth, quite running of the bike.
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  6. phq2m8

    phq2m8 Adventurer

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    For anyone else who might want to do the same at some point.
    Well i got a reply from w650shop, start with 0 spacers.
    So jets are in now and filters are on, will update this post when i finish building bike (garage find in rough cosmetic shape).
    The stock needles are different sizes to allow for bad airflow through original airbox,
    so i was thinking i might just shim the longer one so they match if it runs funny (not sure if anyone has tried this).
    #6
  7. shovelstrokeed

    shovelstrokeed Long timer

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    Here is a little guide line for you on jetting and the like. It is very generic but should help with your thinking.
    Starting at the bottom of the rev range and working toward the top of the tachometer.
    Idle jet and either idle air bleed or idle mixture screw control the basic idle mixture between themselves and the throttle stop plus the slide cutaway which is the notch on the entry side of the slide. Usually arc shaped. Turning the idle mixture screw in or out governs the amount of air mixed with the fuel coming from the idle jet or governs the amount of fuel flowing though the idle jet and into the carb throat.
    Slide cutaway, combined with the idle mixture setting covers the response to very small throttle openings up to about 1/8 throttle. Smaller, or lower, slide cutaway will richen the mixture slightly. This is tender and a PITA to get right as well as expensive. I have been known to slightly sand the flat part of the slide down to obtain a richer cutaway if I cannot get a good transition into that light throttle range. It is really important for a smooth pull away from a stop.
    On most modern carbs, there is an accelerator pump and an emulsion tube that help with this but usually only above 1/8 throttle opening.
    Starting at about 1/8 to 1/4 throttle, the needle and main jet come into play. Needles have all sorts of tapers to allow tuning in this range. Fatter needle for a given throttle opening means a leaner mixture. Remember that the needle interacts with the main jet so needle, higher needle position means a richer mixture as does a richer (larger main jet).
    Finally, we are at 3/4 throttle and up and this is where the main jet fully takes over and your maximum power is available.

    All of these systems interact to some degree and there can be a bunch of overlap between the functions of the different systems. Here is where experts come in. It is actually possible to do all this stuff by eyeball, ear and seat of the pants but it is a whole bunch easier to do it on a dynamometer equipped with a mult-gas analyzer so you can optimize the settings for various loads along with optimum ignition timing settings. An eddy current dyno is also much easier to work with than the more traditional and cheaper inertial types as the engine can run at steady state with varying load to optimize both ignition curve and carb settings.

    This is how the factory set up your carbs and ignition on the bike. Of course they do it for general running around at something like sea level to a couple thousand feet above same. A carb is a wonderfully flexible instrument for this. EFI systems are much more tunable of course with all the extra sensors but it takes even more knowledge and instrumentation to really get things right.
    #7
  8. twinsig

    twinsig Been here awhile

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    #8