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1970s Honda CB100 Won't run unless Wide Open

  1. #1
    Lifer snwbrdr435's Avatar
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    1970s Honda CB100 Won't run unless Wide Open


    Just got a free CB100, it's sat in an airplane hanger for the last 20 years and has around 5k miles on it. Its gotten a brand new carb put on and still wont idle, starts and runs at wide open fine. Any ideas?

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    Lifer Kurlon's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Jetting miles off, float height miles off, air leak?

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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Air leak.

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  4. #4
    Lifer snwbrdr435's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Quote Originally Posted by Degsy View Post
    Air leak.

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    I was told all the fuel lines have been replaced etc, I'll give it a look over when I go to pull it out of the hanger tomorrow night and let you guys know what I find.

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  5. #5
    Soul Rider Paul_E_D's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Points?

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  6. #6
    Member ducatirdr's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    First thing is to replace the intake boot from the carb to the engine. You can test this by running the bike and pointing a garden hose of running water at the boot and listening to the engine. If the sound changes it's leaking. A 40 year old piece of rubber is probably spent.

    Is it a brand new carb? If it's a rebuilt be sure to check the jets. If new make sure it came with the same jets as the ones in your original carb. They usually are clearly marked.

    Google around and see if you can find how to video or threads on the subject.

    CB100/CB125 Hybrid aka CB112.5

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    Last edited by ducatirdr; 10-25-18 at 07:10 AM.
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  7. #7
    Senior Member capitalcrew's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    I would use a bit of starting fluid / ether instead of water personally.

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  8. #8
    Lifer snwbrdr435's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Quote Originally Posted by ducatirdr View Post
    First thing is to replace the intake boot from the carb to the engine. You can test this by running the bike and pointing a garden hose of running water at the boot and listening to the engine. If the sound changes it's leaking. A 40 year old piece of rubber is probably spent.

    Is it a brand new carb? If it's a rebuilt be sure to check the jets. If new make sure it came with the same jets as the ones in your original carb. They usually are clearly marked.

    Google around and see if you can find how to video or threads on the subject.

    CB100/CB125 Hybrid aka CB112.5

    It's a brand new carb, there are also two other brand new ones in boxes. Still have to go grab the thing, but it would be great to rip around before it snows all next week.

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  9. #9

    Re: 1970s Honda CB100 Won't run unless Wide Open

    Iíve always used the gas from an unlighted torch to find vacuum leaks on hoses or cracks in intake manifolds for cars. You slowly move the torch head along side the hoses, fittings and manifold. When the engine rpm changes youíve likely found a leak. Thereís usually more than one. I could see it being a bit trickier with a bike as the air filter intake is quite close. Iíd probably try using a small hand held propane torch as the nozzle is pretty small and easy to direct into tight spaces. If using a cutting torch just use the volatile gas, not the oxygen and just a bit is all that's needed.

    I suspect water would work but it might be a bitch if it sucks too much in and stalls.

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  10. #10
    Your Father csmutty's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Quote Originally Posted by 02redhawk View Post
    I’ve always used the gas from an unlighted torch to find vacuum leaks on hoses or cracks in intake manifolds for cars. You slowly move the torch head along side the hoses, fittings and manifold. When the engine rpm changes you’ve likely found a leak. There’s usually more than one. I could see it being a bit trickier with a bike as the air filter intake is quite close. I’d probably try using a small hand held propane torch as the nozzle is pretty small and easy to direct into tight spaces. If using a cutting torch just use the volatile gas, not the oxygen and just a bit is all that's needed.

    I suspect water would work but it might be a bitch if it sucks too much in and stalls.
    Water would be the best method IMO. This is because it has a small molecular size compared to other molecules which would allow it to slip through cracks that might not be detected by larger molecules.

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  11. #11

    Re: 1970s Honda CB100 Won't run unless Wide Open

    Just run it pinned all the time

    Does it stay running on starter fluid

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    Lifer backinthesaddle's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Quote Originally Posted by breakdirt916 View Post
    Just run it pinned all the time

    Does it stay running on starter fluid
    Good point it is a 100. Either totally on the gas or off.

    Actually cool. Loved all those Honda CB and CL and SL in the 70s. So many displacements.

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  13. #13
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    I have built/rebuilt several of these. They are not super complex but need to be set up properly.


    It will need the right fuel mix - if that is a China carb there is suspect #1. Make sure the tank/ fuel tap are clean. Clean and adjust the points - bonus points if you replace them with OEM (see carb reference above). Make sure you have a fresh battery.

    Check also that you have no restrictions in the airbox. In my experience air leaks caused hanging revs and wild idle speeds - check the basics first. My $ is on battery/points.

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    Last edited by klp; 11-05-18 at 04:08 PM.

  14. #14
    Lifer Stromper's Avatar
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    Re: 1970s Honda CB100 Won't run unless Wide Open

    Fleabay new carb for $20

    Smear/coat carb boot with black silicone

    Its a 100 should only run wide open

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