82 CB900F Carb issues
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82 CB900F Carb issues

This is a discussion on 82 CB900F Carb issues within the Technical forums, part of the Caferacer.net Forums category; I've been having a huge issue with my carbs. Giving it a little throttle, in neutral of course, it revs fine and back down. Give ...

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  1. #1
    Junior Member EofA's Avatar
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    82 CB900F Carb issues

    I've been having a huge issue with my carbs. Giving it a little throttle, in neutral of course, it revs fine and back down. Give it more throttle and it continues to rev up resulting in me having to shut the bike off. Wait a few seconds, then start it back up... back to idle fine. Mind you it's not sticking or hanging, it's reving up and doesn't stop increasing until I have to shut the bike completely down.

    Here's some background.

    - New push/pull cables, lubed up, routed properly with no kinks. Cables even have some slack. Throttle snaps back after release with no hangups or snags.
    - New OEM throttle assembly
    - Took apart carbs for a good cleaning
    - Cleaned all jets and openings everywhere
    - No air leaks in manifolds, I tested that already
    - Idle is stable and carbs have been sync'd
    - Main jet is a 110 and secondary is a 68. Previous owner had a big bore kit done to it.
    - I'm also running pods and not the stock airbox. (Please... those of you who hate the pods... I don't need the comments. Not very helpful.)
    - pilots, lightly screwed in, then unscrewed 2 1/2 turns. No backfires or pops.
    - 4 to 1 pipe with a Supertrapp

    The only thing I can think of right now is that it's the diaphram in the accelerator pump. Or maybe because it's missing the little rubber bellows? I know some of you will say.... "It's running too lean..." Yeah I get that, but should I try getting a 120, 130 or 140 main jet? Maybe just shim up the needle with some washers?

    Anyone have any ideas?
    Eric

  2. #2
    Senior Member Geeto67's Avatar
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    quote:Originally posted by EofA

    - I'm also running pods and not the stock airbox. (Please... those of you who hate the pods... I don't need the comments. Not very helpful.)
    Here's your problem right there. At least if it is the direct problem it is certainly making it worse. Your attitude isn't helping things, I am sure a half dozen people warned you about doing it with these carbs as it is above average in difficulty based on the carb design alone.

    I have never heard of an accelerator pump sticking open. usually when the diaphragm ruptures the pump fails to work. I suppose it is possible for a dirphram to stick open if it wasn't cleaned properly.

    sounds to me like you have sticking slides in the bore. take apart, reclean, grease the slides.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
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  3. #3
    Senior Member Geeto67's Avatar
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    quote:Originally posted by EofA

    - I'm also running pods and not the stock airbox. (Please... those of you who hate the pods... I don't need the comments. Not very helpful.)
    Here's your problem right there. At least if it is the direct problem it is certainly making it worse. Your attitude isn't helping things, I am sure a half dozen people warned you about doing it with these carbs as it is above average in difficulty based on the carb design alone.

    I have never heard of an accelerator pump sticking open. usually when the diaphragm ruptures the pump fails to work. I suppose it is possible for a dirphram to stick open if it wasn't cleaned properly.

    sounds to me like you have sticking slides in the bore. take apart, reclean, grease the slides.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

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  5. #4
    Senior Member Geeto67's Avatar
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    quote:Originally posted by EofA
    Give it more throttle and it continues to rev up resulting in me having to shut the bike off. Wait a few seconds, then start it back up... back to idle fine. Mind you it's not sticking or hanging, it's reving up and doesn't stop increasing until I have to shut the bike completely down.
    this sounds to me like you have no idea how your carbs work. you have CV carbs - constant velocity, ie the slide is controlled by vacuum and not the cable.

    time for a lesson. The throttle cable does not control the slide in the carb (unless you put on aftermarket carbs and didn't tell us), it controls a butterfly valve in the carb that regulates the amount of vacuum the engine draws. as you open the butterfly valve the amount of air going into the vacuum ports increase and the resulting vacuum pulls the slide up. what it sounds like to me is that your carbs have an airleak somewhere or the slide is getting jammed in the body in because after you close the butterfly valve the engine continues to either draw the slide up further or hold it open in position.

    so your comments about nothing being stuck are worthless because they may no be true, just cus the throttle cable moves doesn't mean something ain't jammed.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  6. #5
    Senior Member Geeto67's Avatar
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    quote:Originally posted by EofA
    Give it more throttle and it continues to rev up resulting in me having to shut the bike off. Wait a few seconds, then start it back up... back to idle fine. Mind you it's not sticking or hanging, it's reving up and doesn't stop increasing until I have to shut the bike completely down.
    this sounds to me like you have no idea how your carbs work. you have CV carbs - constant velocity, ie the slide is controlled by vacuum and not the cable.

    time for a lesson. The throttle cable does not control the slide in the carb (unless you put on aftermarket carbs and didn't tell us), it controls a butterfly valve in the carb that regulates the amount of vacuum the engine draws. as you open the butterfly valve the amount of air going into the vacuum ports increase and the resulting vacuum pulls the slide up. what it sounds like to me is that your carbs have an airleak somewhere or the slide is getting jammed in the body in because after you close the butterfly valve the engine continues to either draw the slide up further or hold it open in position.

    so your comments about nothing being stuck are worthless because they may no be true, just cus the throttle cable moves doesn't mean something ain't jammed.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  7. #6
    Junior Member EofA's Avatar
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    Thanks for the input, talk about attitude.... ha!

    You might be right about this slides. Any suggestions on what type of lube to put on them?

    My description of nothing sticking was mostly related to cables and throttle assembly. I will check the slides tonight. I'm getting new gaskets and o-rings along with new accelerator and air cut off diaphrams. 2 of them look a bit brittle, so just to be sure.
    Eric

  8. #7
    Junior Member EofA's Avatar
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    Thanks for the input, talk about attitude.... ha!

    You might be right about this slides. Any suggestions on what type of lube to put on them?

    My description of nothing sticking was mostly related to cables and throttle assembly. I will check the slides tonight. I'm getting new gaskets and o-rings along with new accelerator and air cut off diaphrams. 2 of them look a bit brittle, so just to be sure.
    Eric

  9. #8
    Senior Member hillsy's Avatar
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    quote:Originally posted by EofA

    You might be right about this slides. Any suggestions on what type of lube to put on them?
    None. The slides just need to move freely in the carb bores. Any "lube" will be sucked into the motor or attract dust and crap which will just seize up the slides.

    Take the carbs off, remove the tops and slide springs, then lift each slide from the rear (pod) side. When you let them go they should "clunk" back down. If not, you need to take each one out and clean / polish them.

  10. #9
    Senior Member hillsy's Avatar
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    quote:Originally posted by EofA

    You might be right about this slides. Any suggestions on what type of lube to put on them?
    None. The slides just need to move freely in the carb bores. Any "lube" will be sucked into the motor or attract dust and crap which will just seize up the slides.

    Take the carbs off, remove the tops and slide springs, then lift each slide from the rear (pod) side. When you let them go they should "clunk" back down. If not, you need to take each one out and clean / polish them.

  11. #10
    Senior Member Teazer's Avatar
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    Slides should come back down somewhat smoothly and not clunk down hard. If the slides happen to stick in the up position the revs should come back down because as Geeto pointed out, the butterflies control idle speed. With a a stuck slide, the gas velocity through the carb is usually insufficient to pull much fuel out of the needle jet/main jet.

    Most likely culprits are air leaks around the intake boots or between the bots and head or one or more of teh butterflies is not closing. The linkages include small screws and springs. If one or more are missing, that would allow one or more throttle plates to remain open.

    And with pods the jetting will never be correct as people have already indicated. It's not a hate thing. It's fact. much better men than me have come up with some amazing back bleed conversions to get rid of the huge mid range bog on those things.

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