Carb Rebuild
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Thread: Carb Rebuild

  1. #1
    Member knauff13's Avatar
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    Carb Rebuild

    Well, on my way home yesterday on my 78 CB400A, everything was going fine until I was almost home and at a stop light I noticed I was bogging a little almost like I was out of fuel. But that didn't seem likely, as I only had 40 miles on the tach.

    When I got a chance to pull over, I saw a fair amount of gas dripping from the tubes in front of the back tire. I closed the petcock and the dripping stopped pretty quickly. I managed to limp it home on reserve and a prayer.

    Because the petcock seems to be functioning appropriately, this seems like a pretty certain case of a carb issue, right? That's what my research here indicates, anyway.

    I really wasn't having any running issues at all before this (even just before I stopped at that particular light--actually, it's possible the only running issue I had was low gas because of the leak), so I was wondering if there are any high points I should check before I get elbows deep in a carb rebuild. I guess I'd plan to do that this fall anyway, but while I'm somewhat mechanically inclined, I'm not experienced in engine work and I'm reluctant to engage in something that may leave the bike out of order for a month... not because I think it will take that long, but with two young kids, free time comes it small doses.

    In any case, I'm picking up an ultrasonic cleaner, gaskets, and a couple other odds and ends that I think I'll need (if not now, then soon). I've also saved LDR's sticky on carb rebuilding, for which I'm extremely grateful. Here, for anyone else who comes across this: https://www.hondatwins.net/forums/63...tin-carbs.html
    Last edited by knauff13; 05-07-2019 at 12:18 PM.
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  2. #2
    Senior Member Bill H's Avatar
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    LDR carb rebuild is invaluable but an A carb is a little different from the 400s . A cb 400 a FSM will give you the settings for your carbs. I used his sticky to rebuild my carbs then the manual to fine tune them.

    If you havent worked on these carbs before give us a shout maybe we can give you some tips on removal , build and reinstall.

    Bill
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    Member knauff13's Avatar
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    Thanks Bill! Hope you're recovering well from the surgery. I've read some tips on getting them out, so I think I'll start there and report back when I run into any issues. I do have a manual saved somewhere. Good point on that.
    Ben Knauff
    1978 CB400A Hawk Hondamatic

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    Sensei 66Sprint's Avatar
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    Check the bowl overflow tubes first...
    They crack and then can dump enough fuel that the carb jets barely get (sometimes don't get) enough to run the engine.....
    Fortunately, there's an easy fix....Solder them......There is a topic on this.....
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    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
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    Member knauff13's Avatar
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    Got it, thanks. Posting this link about soldering the overflow tube here for my own later reference if that's what I find.

    https://www.hondatwins.net/forums/34...low-tubes.html
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  7. #6
    Member knauff13's Avatar
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    I'm having a heck of a time finding these screws: Stainless Steel Allen Head Screws: 8: 4x12mm, 8: 4x16mm, 4: 5x12mm, 5: 5x16mm, 4: 6x14mm. I'm trying to find them online after striking out at local hardware stores.

    Sorry if this is a dumb question, but does anyone know the thread pitch of these screws?
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  8. #7
    Senior Member Bill H's Avatar
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    I think LDR is on the road. You might shoot dtsmjr8dan a PM and see if he can help with the thread pitch. Fastenal on line will probably have them but searching thru their web site is a pain.

    Leg is doing fine. Ahead of the PT but have been told to back off a little as the muscles need some more time. Getting way behind on the yard work etc.

    Bill
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  9. #8
    Member knauff13's Avatar
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    Bill, great tip on Fastenal. I think I've located them. They're only offering "coarse" threads for these allen/socket heads of any size, so I think that must be it. I know from personal experience PT is more marathon than sprint. Glad to hear you're doing well.
    Last edited by knauff13; 05-08-2019 at 07:28 AM.
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  10. #9
    Member knauff13's Avatar
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    Can you guys see how one float is sitting higher than the other? Would this have caused the carb overflow issue? I checked the bowl overflow pipes and they both look intact. The bowls were actually very clean.
    Attached Thumbnails Attached Thumbnails Carb Rebuild-20190510_212307_1557537987249.jpg   Carb Rebuild-20190510_212334_1557538008332.jpg  
    Ben Knauff
    1978 CB400A Hawk Hondamatic

  11. #10
    Senior Member Polish's Avatar
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    It looks like the right side float is higher. Is it the one leaking?

    Follow the CV carb rebuilt to the letter and you will have perfect carbs. Did the same with mine and soak them in the ultrasonic cleaner as well. It looks daunting to do the carbs, but actually very doable. Good luck!
    JL
    1979 CB 400 T (Cafe Racer)

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