Maintains idle at 1200 RPM but cuts out with any throttle movement
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  1. #1
    Junior Member Jbadcock's Avatar
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    Maintains idle at 1200 RPM but cuts out with any throttle movement

    Learning as I go here and just started this 1978 CB400T after 25 years under a tarp. From the cursory reading of the potential carb issues (most of which I don't understand yet) it looks like the idle circuit is separate from the throttle actuation. Correct? I have cleaned the carbs and re-installed with original parts (maybe that is already expecting too much). New throttle cables and choke cable installed but perhaps not adjusted properly? From what I read here on the forum I still have to synchronize the carbs but I wonder if that would remedy the "stall on throttle" issue?
    Thoughts anyone?
    Last edited by Jbadcock; 10-13-2019 at 07:59 PM.

  2. #2
    Senior Member TOOLS1's Avatar
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    Sounds like you have a vacuum leak. Check the rubber boots between the carbs and head for leaks.
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  3. #3
    Super Moderator J-T's Avatar
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    You could also have blocked carb circuits. By all means check for vacuum leaks first but, if you don't find/fix the problem there, start with this:

    https://www.hondatwins.net/forums/63...tin-carbs.html

    It's unlikely you'll need to replace any hard parts other than going up one size on the primary main jets.

    A bench sync should be adequate to get it running.
    JT
    81 CM400T

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  5. #4
    Junior Member Jbadcock's Avatar
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    I will look into that next. Just now sprayed some quick start around the boots while the engine idled at 1200 and absolutely no change in RPM. I think they are sealed ok.

  6. #5
    Junior Member Jbadcock's Avatar
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    Doing some recommended reading (thanks for the pointers) and now wondering if the float is functioning properly. There seems to be a steady trickle of fuel coming from the overflow tube. I repaired one of the overflow tubes with some JB weld (it had essentially snapped off during the cleaning process) and appeared to be solid when it was re-installed but maybe this is part of the problem? Tried adjusting the fuel mixture screws to factory setting but seemed to have little effect. The RPM jumps up with the choke pulled but still cuts out completely the second I move the throttle. Is this cause for a re-build?
    Last edited by Jbadcock; 11-04-2019 at 06:35 PM.

  7. #6
    Super Moderator J-T's Avatar
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    The cracked/repaired overflow tube should not have any effect unless combined with a leaking float valve resulting in a high fuel level in the bowls. That should not cause your symptoms.

    The lack of response to mixture screw adjustments along with the lack of vacuum leaks leads me back to the carb circuits. A thorough cleaning and re-build following Jim's tutorial would be a good approach. There may be more, but the effort will still pay off.
    JT
    81 CM400T

  8. #7
    Junior Member Jbadcock's Avatar
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    Thanks again JT! Picking up a re-build kit today.

  9. #8
    Super Moderator J-T's Avatar
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    Not an aftermarket re-build kit I hope! Read the tutorial first.

    What you need is the Honda gasket set, possibly Honda float valve needles, any necessary hard parts need to be acquired individually. Jets don't wear out and usually only need to be replaced if damaged or to change sizes.
    JT
    81 CM400T

  10. #9
    Junior Member Jbadcock's Avatar
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    Thanks JT. You are so on top of it! I read Jim's post (which is incredibly detailed) with the caveat about aftermarket kits and I hope to tackle a more comprehensive cleaning and soft part replacement ASAP. I thought I had the thing cleaned up but LDR has waaaaay higher standards. I used a carb cleaner product but I see Pine Sol is an option. What is your solvent of choice?

  11. #10
    Super Moderator J-T's Avatar
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    Personally I use Carb Medic solvent dip. I wouldn't necessarily recommend it for someone who is going to use it just one time but if you do you'll need to take it to a hazardous wast disposal facility to get rid of it. Similarly purchasing an ultrasonic cleaner may not make sense for just a one time use. Pinesol gets good reviews as do a few other common cleaners. Some will eat the metal if left too long so a bit of caution is in order. I find the most valuable tool for cleaning carbs (or a lot of other stuff) is a good source of compressed air. Whatever you use to dissolve the gunk in the carb circuits needs to be blown out.
    JT
    81 CM400T

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