Synchronizing the carbs
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Thread: Synchronizing the carbs

  1. #1
    Senior Member Chippe's Avatar
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    Synchronizing the carbs

    I have synchronized the carbs again with the two bottle method. I do it on a warm engine at idle speed and the water level doesn't move. It's pefectly synchronized at idle speed but if I twist the throttle or change the idle speed a little they're no longer synchronized. Left carb sucks more as the rpm goes up.
    Last edited by Chippe; 06-06-2019 at 02:00 AM.
    Honda CM400 1979

  2. #2
    Super Moderator J-T's Avatar
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    Normal.

    In order to get perfect synchronization throughout the throttle range you would need every other adjustment on the engine to also be perfectly synchronized.

    If it runs good, don't worry about it.
    chaindrivecharlie and Chippe like this.
    JT
    81 CM400T Under re-construction
    82 GL500

  3. #3
    Senior Member Lefty's Avatar
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    Idk if they have twin carb cable adjusters but if so one could have more slack.

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  5. #4
    Super Moderator J-T's Avatar
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    They don't.
    JT
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    Senior Member Lefty's Avatar
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    Ahh, did it today on my dohc, thought maybe

  7. #6
    Senior Member Chippe's Avatar
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    Thank you J-T
    Honda CM400 1979

  8. #7
    Senior Member Chippe's Avatar
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    When synchronized at idle speed it's not perfectly synchronized at higher rpm. If I'm holding the throttle so the rpm is at around 1500-2000 and synchronize it'll be stable at that rpm and above. That means there could be a play in the linkage between the carbs.
    Now I have pretty good sync at around 2000 rpm and above but not so good at 1200 rpm but I think it's better this way. It's nothing I feel, it it's just something I see when the sync tool is connected.

    What do you say about this?
    Last edited by Chippe; 06-07-2019 at 01:27 PM.
    Honda CM400 1979

  9. #8
    Senior Member ancientdad's Avatar
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    I've owned twins, 4 cylinders and 6 cylinders and I've "put up with" slightly imperfect idle on many of them over the years because that's all it is - idle. My first 6 cylinder, after 20,000 miles of pretty hard riding, did not idle perfectly... it was a little rough and that made the clutch basket noise more prominent than it might have been, but considering the depth of involvement to make it any reasonable amount better, and considering the tiny little benefit that would have brought, I didn't bother. The bike ran fantastically off idle and everywhere else - so it just wasn't worth the trouble. And when I (stupidly) sold the bike after about 4 years of ownership, the buyer didn't notice anything bad about it either...
    Tom

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  10. #9
    Senior Member Chippe's Avatar
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    Ok, cool. Yes, I guess idle isn't important as long as it idles. The bike is supposed to be in sync from idle rpm and above but it seems like there is a play in the linkage so it's better to have tension in the throttle wire and then synchronize. Just something to think about if I'm right about it.

    Clutch basket noise, what does it sound like?
    I noticed something I haven't payed attention to before. I hear a knocking sound in neutral but not if I pull the clutch. Btw, I also heard this when the carbs was in sync at idle speed.
    Honda CM400 1979

  11. #10
    Senior Member ancientdad's Avatar
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    The primary drives on many of these older engines can be a little noisy... the ones with straight-cut gears (not 2 sets staggered) are noisier, and in some cases the clutch baskets come slightly loose on the rivets that hold them together and that can add to the noise. If the noise decreases when you pull in the clutch, that's usually a sign it's the primary drive or clutch basket making the sound... generally nothing to worry about, only when the rivets in the basket are too loose, but the noise would be a lot louder then and you'd hear it well enough to know there would be reason for concern
    chaindrivecharlie and Chippe like this.
    Tom

    Ride along at the drag strip - https://www.youtube.com/watch?v=20jFPazXlvU



    running points... because I'm too old for mysteries that begin with pushing

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