Jetting Guru's! Dual 3 jet Keihin CDK2 40's help! - Page 6
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  1. #51
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Quote Originally Posted by Bob Gruen View Post
    Another problem I am having is simply reaching the idle screw on the left carb. The best solution I have found so far is putting primer tubing on it
    and turning it with my fingertips. I don't have a whole lot of confidence in it as an accurate method but I can't get a screwdriver head anywhere near it.
    I use a Snap-On small blade (~3/16") screw driver with a long ~12" round shank
    to adjust both front and rear pilot screws on the X2. It isn't easy, but it works.

    I did have an idea for how to make one from a Kawasaki speedometer cable. The
    pictures explain my idea.

    A more practical tool may be a flexible 1/4" drive screw driver with a small recessed
    flat blade screw driver 1/4" socket. I used to use this set-up years ago, but I lost
    the 1/4" socket. But I still have 2 flexible 1/4" drive screw drivers, one short, and
    one long. They really come in handy while working in the tight spaces of a jet ski,
    or sea doo.

    Screwdriver, Recessed Blade Carburetor, Flat Tip, 9/32" x 1/32"

    Driver, Flexible, Long Cable, Standard Handle, 9-3/16

    6 3/4" Standard Flexible Driver


    Bill M.
    Attached Images Attached Images
    Horsepower == Speed, RPM != Speed



  2. #52
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Quote Originally Posted by Bob Gruen View Post
    There is no power off of idle, it stumbles and even dies. A few shots from the primer are required to get it running so it is a severe lean condition.
    The pop-off pressure is at the end of the Mikuni Gauge's scale at 26ish psi, and the pivot is set properly. I am running: 72 / 50 / 135.
    Quote Originally Posted by Bob Gruen View Post
    Had my first successful run today, pretty sure the fuel selector switch was part of the problem. Ended up with the low screws at 1 & 3/4 turns
    out and the highs at 1 & 1/2 out.
    Were your pilot screw adjustments the result of the idle drop test, or that
    was just where the engine ran the best?

    Your adjustments are very close to what Miguel ended up with. The 1 3/4's
    works out to a 85 to 90 low jet, and the mains should be ~145.

    Miguel ended up with 90 lows, 45 mid, 140 mains.

    Quote Originally Posted by Bob Gruen View Post
    Finally, the boat is just too loud.
    Weld a bung to your waterbox and install a a 90 deg, brass fitting for 3/8" hose.
    Then direct a water line you have routed as a pisser, to the waterbox instead.

    Quote Originally Posted by Bob Gruen View Post
    Looking at the tuning documentation I see the wisdom in this for the low screws and making the high screws match makes sense. I may do a full
    write-up when I am confident I understand everything that is going on here (or maybe Bill will beat me to it).
    I have already written a lot on jetting carbs, power tuning, and using the idle
    drop test. The idle drop test is important, because it creates a baseline for the
    pilot screw, spring, seat, low jet and idle. After the low speed is set, you can
    easily power-tune the hi-speed screws, and make main jet adjustments, or by
    using the sum of the low and main jet, you can estimate the new main jet.

    My recommendations for SBN low & main jet changes based on spring rate
    works consistently for single carbs using 2.0 needle and seats, and dual
    carbs using 1.5 needle and seats. Unless you need to use larger needles &
    seats for a specific purpose (like Novi jetting style) there is no need, IMO,
    for seats larger than 1.5 on duals.

    On the Keihin carbs, my rules for 1 jet for every 1/4 turn on a screw is a good
    starting point.

    On 3 jet carbs, the sum of all the jets doesn't quiet work on these carbs, but
    I believe it works well enough to compare to other 3 jet carbs, but not a 2 jet
    carb. The mid-range jet should be compared to an accelerator-pump. If you watch
    the mid-range jet work (after it has been drilled), it will spray fuel to the oppisite
    side of the carb as the throttle butterfly opens the port. A 45 low jet should be
    considered as the minimum mid-range jet size.

    I modified your Keihin CDK-2 Circuit chart. I also supplied a picture of a set of
    Suzuki GSX-R carbs, with special emphasis on what a pilot circuit is, and why
    our carb screws are not really low screws at all.

    I have more planed, but I didn't have time for it today.



    Bill M.

    Bob, one other thing? Was the fuel pet**** clogged? If bypassing it helped,
    why did it help? A simple thing like the pet**** elevation is no big deal for a
    properly working fuel pump. BM.
    Attached Images Attached Images
    Horsepower == Speed, RPM != Speed



  3. #53
    I dream skis Bob Gruen's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Bill,

    I don't believe your correction to the diagram I did is accurate.

    First, the mid jet transfer hole is parallel with the upper most transfer holes from the low speed circuit, therefor it is experiencing the same vacuum and should taper off at the same time the low speed jet does. You showed a pic in a post somewhere of a CDK2 carb that had the mid circuit drilled with two holes, the second being higher that the one I have (pictured in this thread). That second hole may extend the operation of the mid jet a touch further into the throttle opening, but not by much as the vacuum at the edge of the throat is really falling away at that point.

    Second, there is nothing in the carb that creates an 'on' point for the mid jet, so having it start all at once is definitely not correct. In hindsight the slope of the leading line should probably be steeper than I drew. The nature of how the vacuum pattern ramps up as the butterfly valve edge approached the transfer is probably not even linear. How about this revision:

    cal_chartcdk.3Jet.gif

    Effectiveness is kind of a funny concept to plot on the axis. Actual fuel delivery the size of the jets would alter the slopes of the various triangles. For example, the slope of the mid jet should be right at 2/3rds of the slow jet (50 vs 75ish), while the high speed jet should be almost three times as steep at 135.

    I looked at getting various tools to try to reach the screw but gave up. I don't see how anyone can do it short of a modded Snap-On 90 degree carb tool that Wade down in Sarasota, Fl once showed me ($$$). The knurled knob extension is promising though. I have to groove and paint witness marks then disassemble the carbs from the manifold to align for the glue-up.

    I have not really looked at the fuel valve just yet. Another option is to add a second fuel pump to the second carb body, but that would involve tapping out the holes for screws, pulling a pressed in fitting and drilling and pressing it back in in a different position. No going back once that is started.

    Bob
    Last edited by Bob Gruen; 09-28-2016 at 11:08 AM.
    750 X2 build thread: http://www.pwctoday.com/showthread.php?t=463139
    Twin 550SXs build thread: http://www.pwctoday.com/showthread.php?t=468310
    Trailer mod thread: http://www.pwctoday.com/showthread.php?t=467191
    Keihin 3 Jet tuning thread: http://www.pwctoday.com/showthread.php?t=400156

    My website and links to my film, The Hobby Stop: http://www.rannugmedia.com

    Classic 911 website: http://members.rennlist.com/bgruen/


  4. #54
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Bob, that chart is trash.

    The low speed circuit doesn't start at 0%. The hi-speed doesn't
    reach down to 1/4 throttle. The mid-range circuit comes in all at
    once and squirts all the way to the other side of the carb bore.
    That is why I said it was like a accelerator pump.

    I doubt you will be able to tell when the mid is too lean, and when
    it is too rich, it will most likely cause the powerband to stall (blubber)
    about 1/4 to ~1/3 throttle, and then clear up and continue accelerating.

    The low speed, and mid-range circuit continue to flow fuel to wide
    open throttle. Their effectiveness decreases substantially, but any
    changes (small or large) to the low or mid will effect the hi-speed
    screw setting because both circuits contribute to the sum of the
    jets and screws.

    Mikuni and Walbro sold carbs with mid-range circuits back in the 80's.

    I am going to reorder the snap-on socket. But, like I said, I use a
    long screw driver and that works. Perhaps you can add a allen head
    bolt to your primer tubing, and then a ~3mm T-head allen wrench
    will do the job.

    I don't see the need for a second pump. The only thing that makes
    sense (if everything is mechanically sound) is fuel volume for dual
    carbs. We do run dual Keihin carbs with dual fuel pumps on the super-
    cylinder X2. If you want to run Keihins on a hi-hp X2, I recommend
    Polaris 1050 triple carbs, converted to duals (but it not a simple con-
    version).

    I originally wrote a much longer response, that details why that graph
    is junk, and more on carb design. But this is the short version .


    Bill M.
    Last edited by wmazz; 09-29-2016 at 01:06 AM.
    Horsepower == Speed, RPM != Speed



  5. #55
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    As an example of a carb that comes close to that chart (If you modify
    where the low speed starts, and raise the pilot circuit a bit). The Sea-
    Doo 720 carbs. Dual 38mm SBI carbs use all the design tricks to
    achieve such a low starting point for the mains.


    Reverse jetting, or your carbs with ~85 low jet will extend the low jets
    range.

    A Buckshot is an example of a carb that has a very short main jet range,
    and a longer lean spot between the end of the low jets to the beginning
    of the main jets. Using reverse jetting to extend the low jet range, helps
    to reduce the lean spot.

    The mid-range circuit helps to bridge the lean spot between the lows ending
    and the mains beginning, similar to using reverse jetting.

    Now you have two examples: The 38mm 720 carbs are the best, and the
    modified 44-46mm sbn Buckshots are the worst. The majority of modified
    carbs are similar to the Buckshots.

    I hope this is more helpful than my other post. BM.
    Last edited by wmazz; 09-29-2016 at 03:51 PM.
    Horsepower == Speed, RPM != Speed



  6. #56
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Sorry, I should have done this the first time.

    Quote Originally Posted by Bob Gruen View Post
    First, the mid jet transfer hole is parallel with the upper most transfer holes from the low speed circuit, therefor it is experiencing the same
    vacuum and should taper off at the same time the low speed jet does.
    From what I can tell from observing the low speed by-pass holes while I
    powertune the carb. The low speed circuit begins 1 hole at a time as they
    are uncovered. Then, (depending on the location and diameter of the upper
    by-pass holes) the upper by-pass holes will shoot across the throttle butterfly
    before the upper port opens, and then continue to supply fuel after the
    throttle butterfly has opened the upper by-pass holes.

    Quote Originally Posted by Bob Gruen View Post
    Second, there is nothing in the carb that creates an 'on' point for the mid jet, so having it start all at once is definitely not correct.
    The throttle butterfly has one side that rotates downward, and one side that
    rotates upward.

    When the side that is rotating downward allows enough vacuum to pass by
    the throttle butterfly, The upper low-speed by-pass holes, and the mid-range
    circuit will start before the throttle butterfly opens the holes.

    When that happens they will shoot fuel all the way across the top of the
    throttle butterfly.

    Then continue to supply fuel after the throttle butterfly has opened the
    mid-range circuit.



    Bill M.
    Last edited by wmazz; 09-29-2016 at 04:12 PM.
    Horsepower == Speed, RPM != Speed



  7. #57
    I dream skis Bob Gruen's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    OK, I understand that this is a phenomena that you have witnessed first hand. Looking at the physics involved I just don't see what could cause it to behave the way you describe.

    As for my situation, I can't find nickel plated jets below 50, and I am unwilling to run the non-plated off brand jets. Because the low and mid range feel good I'm not going to change any of it. I could increase the main jet, but since my top end screw is in an acceptable range I think I'll just let it be.

    Bob
    750 X2 build thread: http://www.pwctoday.com/showthread.php?t=463139
    Twin 550SXs build thread: http://www.pwctoday.com/showthread.php?t=468310
    Trailer mod thread: http://www.pwctoday.com/showthread.php?t=467191
    Keihin 3 Jet tuning thread: http://www.pwctoday.com/showthread.php?t=400156

    My website and links to my film, The Hobby Stop: http://www.rannugmedia.com

    Classic 911 website: http://members.rennlist.com/bgruen/


  8. #58
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Quote Originally Posted by Bob Gruen View Post
    Looking at the physics involved I just don't see what could cause it to behave the way you describe.
    I dropped my physics class, but I know what basic forces are acting inside a carb.
    If you take the time to learn the physics of reverse jetting, then you can apply what
    you learned to the mid-range circuit!

    Quote Originally Posted by Bob Gruen View Post
    I can't find nickel plated jets below 50, and
    Bob I never suggested a #45.

    I suggested to anyone reading this not to use any jet smaller than a #45.

    Quote Originally Posted by Bob Gruen View Post
    I am unwilling to run the non-plated off brand jets.
    If anyone wants a #45, they can be purchased at a Kawasaki dealer.

    If anyone wants genuine Keihin jets, purchase them from Sudco. But they are ~$7 ea.


    But I suggest anyone to drill their own jet sizes #30 to #160
    Last edited by wmazz; 10-01-2016 at 02:22 AM.
    Horsepower == Speed, RPM != Speed



  9. #59
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    Hey guys, just wanted to say thanks for solving a big headache. I bought the 3 jet carb and ended up only having to switch the mid to a 50 jet, 1 turn low and 1 turn on high. She lights right up! Using stock flame arrestor, and a TDR waterbox.

  10. #60
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    Re: Jetting Guru's! Dual 3 jet Keihin CDK2 40's help!

    s-l1600.jpg

    Say, I picked up a set of dual carbs from a 98 750 STX. It's got 145/72/45 I think for jets. I'm using a small pin 750 with a factory limited pipe. Any suggestions for jetting? Factory recommended 125 main, 75 pilot. How would this convert to a 3 jet set of carbs?

    Also, what is this tiny fuel line that connects from the side of the fuel bowl? Accelerator pump? Where does it hook up to?

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