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Thread: Syncing carbs.

  1. #1
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    Default Syncing carbs.

    I thought I’d make one more video before tearing the bike apart. This one deals with carb syncing with a manometer.


    This manometer has a relatively large body of water, and the vacuum lines are fitted with a .6mm jet. This has the effect of slowing down the movement of water stabilising the readings and making the device extremely accurate. Just a miniscule turn of either idle screw or throttle adjustment screw greatly effects the fluid levels.


    I have great trouble with my fingers, so you'll notice me changing hands frequently trying to turn the adjustment screws. I also turned the adjustment screws the wrong way a few times, putting the levels out.


    https://www.youtube.com/watch?v=9-iw8Z2I5o8

  2. #2
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    Back when I owned bikes I just used a flexible plastic tube - one end in the ear and the other end held to the carb throat to 'listen' to the suction at each carb throat, adjust until each has the same sound, this works well and you can get them real close, back in 1971 my big bro bought a Torana XU1 - I balanced the carbs using a tube, a few months later he had the car tuned at a workshop called Maxwell Dynotune - they said the carbs were spot on for balance. My school mate's father a fitter and chief engineer on one of the Port Adelaide steam tugs taught me this.

  3. #3
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    Very good. As long as the plates are 100% open at full throttle that's all that matters!!!

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    Yep, there's many ways of syncing carbs, tube in each ear, popsicle sticks, vacuum gauges, a loop of plastic tube. I've used all these methods. But time moves on and with that comes improvements in the way we do things, it's now 2022, not the 50s, 60s or 70s.
    Lots of people liked the horse and cart, but I prefer what we have now, preferably without the two men with red flags running along in front of my horseless carriage to warn everyone!

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    Quote Originally Posted by Mk1_Oz View Post
    Very good. As long as the plates are 100% open at full throttle that's all that matters!!!
    What plates?

  6. #6
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    Quote Originally Posted by Hoggo1951 View Post
    What plates?
    Butterflies id say , lol . I had tripple SU's on a XU1 motor in a HQ , the only way i could start it cold was open the bonnet and pour a litre of fuel into them.

  7. #7
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    The XS650 had BS38s on it as standard (CV carbs). They're now in the bin replaced with VM34 roundslides.
    Attached Images Attached Images

  8. #8
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    Quote Originally Posted by Hoggo1951 View Post
    What plates?
    throttle plates/butterflies

  9. #9
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    Quote Originally Posted by Hoggo1951 View Post
    Yep, there's many ways of syncing carbs, tube in each ear, popsicle sticks, vacuum gauges, a loop of plastic tube. I've used all these methods. But time moves on and with that comes improvements in the way we do things, it's now 2022, not the 50s, 60s or 70s.
    Lots of people liked the horse and cart, but I prefer what we have now, preferably without the two men with red flags running along in front of my horseless carriage to warn everyone!
    The popsicle stick method intrigues me - I've never heard of it - was it just a tongue in cheek joke or is there really a popsicle stick method ? if so how does it work.

  10. #10
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    Quote Originally Posted by Hoggo1951 View Post
    What plates?
    You put a popsicle stick under each slide, hang it out the rear and adjust until they are level.

  11. #11
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    Don't you disconnect all mechanical connection between all the carbies, adjust the butterflies at idle then reconnect the shafts between the carbies at idle? At that point they are synchronised.
    CHRIS

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    That would only sync the carbs at idle, at all other engine speeds the carbs would not be synced..

    There are no butterfly's on VM carbs.

    Mechanical connections have no bearing on syncing carbs at idle, only idle screws do. So disconnecting any mechanical connections will have no effect. At anything above idle the mechanical connections comes into play. It is the mechanical connections that are adjusted to sync carbs at anything above idle speed.

    In the case of slide carbs, such as the VM, the mechanical connections raise or lower the slides. In CVs, such as Mikuni VMs, SUs and the like, the mechanical connections open or close the butterflies, thus increasing or decreasing vacuum witch raise or lower the slides.

  13. #13
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    Quote Originally Posted by Hoggo1951 View Post
    That would only sync the carbs at idle, at all other engine speeds the carbs would not be synced..
    That's good news, every multi car set up done since the beginning of time is wrong.
    CHRIS

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    So you're saying carbs only need to be synced at idle???

  15. #15
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    Quote Originally Posted by Hoggo1951 View Post
    So you're saying carbs only need to be synced at idle???
    Certainly not. Get them synched at idle and the butterflies stay synched right through to WOT. Mixture adjustment is then done as needed whether by jetting or needle replacement. The jetting of a Weber will not be possible without good mixture monitoring but a Reece Fish is another animal altogether. Have you no need to adjust the jetting to match the motor? Each cylinder will require individual jetting to get it spot on and these days it is best done by O2 sensors and exhaust temps.
    CHRIS

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