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Thread: '81 CB250RS won't rev down

  1. #1
    Join Date
    23rd August 2021 - 20:52
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    '81 Honda CB250RS
    Location
    Auckland
    Posts
    9

    '81 CB250RS won't rev down

    Hi all
    My CB250RS has recently started having trouble reducing revs when i let go of the throttle - it will stay at whatever it was at for a bit which makes it very hard to ride and shift.
    Sometimes (though less often) it revs up slightly by itself, or is generally lurchy - might just be my riding

    I slackened the throttle cables slightly, and noticed that the return cable has a lot of slack in it and doesn't appear adjustable. However, the little rotating bit on the carb returns home fine under spring power (yet the revs stay high), so I doubt the cables are at fault.

    Obvious answer would be an air leak in the boot between the carb and head, so I'll check that. I've read that people suggest spraying with brake cleaner and see if revs change, is that actually worth doing?

    There are some other recent developments that could (?) be related:

    The front left rocker cover bolt is stripped and is now jiggling around in its seat. Oil and air bubbles are coming out of the small gap between the rocker cover and head, so could this somehow be causing my revving problem? Is this worth fixing? I presume helicoil it, any tips for that?

    Also, there's a bubble stuck under the inline fuel filter, could this be doing something? The bike still pulls fine up hills etc so it gets adequate fuel, so I doubt that this is the issue.

    Thanks

  2. #2
    Join Date
    20th January 2010 - 14:41
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    husaberg
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    Quote Originally Posted by matthewb77 View Post
    Hi all
    My CB250RS has recently started having trouble reducing revs when i let go of the throttle - it will stay at whatever it was at for a bit which makes it very hard to ride and shift.
    Sometimes (though less often) it revs up slightly by itself, or is generally lurchy - might just be my riding

    I slackened the throttle cables slightly, and noticed that the return cable has a lot of slack in it and doesn't appear adjustable. However, the little rotating bit on the carb returns home fine under spring power (yet the revs stay high), so I doubt the cables are at fault.

    Obvious answer would be an air leak in the boot between the carb and head, so I'll check that. I've read that people suggest spraying with brake cleaner and see if revs change, is that actually worth doing?

    There are some other recent developments that could (?) be related:

    The front left rocker cover bolt is stripped and is now jiggling around in its seat. Oil and air bubbles are coming out of the small gap between the rocker cover and head, so could this somehow be causing my revving problem? Is this worth fixing? I presume helicoil it, any tips for that?

    Also, there's a bubble stuck under the inline fuel filter, could this be doing something? The bike still pulls fine up hills etc so it gets adequate fuel, so I doubt that this is the issue.

    Thanks
    if its lean it can have a hanging idle esp if its on the pilot circuit ie your pilot jet could be blocked
    also on the carb (if its the same as a XL250 of the same era it had a spring on the push pull lpart of the carb plate the cable went in.i have a complete carb for a XL250S or RC complete with accelerator pump, open to offers.
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    or your slide coulfd be damaged
    or your cables could be buggered
    buy a cable luber or make up a funnel with blue tack and lube chain lube or other lube
    Click image for larger version. 

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    Quote Originally Posted by Katman View Post
    I reminder distinctly .




    Kinky is using a feather. Perverted is using the whole chicken

  3. #3
    Join Date
    23rd August 2021 - 20:52
    Bike
    '81 Honda CB250RS
    Location
    Auckland
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    9
    Just went out to see if putting the choke on would make it less lean. This didn't do much, and I ended up resetting the idle adjuster which appears to have fixed it. I thought I already had this pretty well set, but I guess I might have bumped it while fiddling with my cables. Anyway, I'll go for a ride tomorrow and see if the problem comes back.

    Question about helicoiling the rocker box cover stands - do I bother?

    Thanks

  4. #4
    Join Date
    20th January 2010 - 14:41
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    This bit?
    Click image for larger version. 

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    try and get another rocker cover is likely cheaper
    unless you can do it yourself
    Quote Originally Posted by Katman View Post
    I reminder distinctly .




    Kinky is using a feather. Perverted is using the whole chicken

  5. #5
    Join Date
    23rd August 2021 - 20:52
    Bike
    '81 Honda CB250RS
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    Auckland
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    Nah one of the vertical bolts running into the head. Part 26 ish in this diagram
    Would probably do it myself unless there's a good reason not to.
    https://images.cmsnl.com/img/partsli...41e01_5215.gif

    Also realised I cleared my little air bubble under the filter when mucking around the other night. I suppose that could have been making it lean, causing the hanging idle problem.

  6. #6
    Join Date
    25th March 2004 - 17:22
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    RZ496/Street 765RS/GasGas/ etc etc
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    Wellington. . ok the hutt
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    Buy a helicol kit. Won't be the last time you use it. Just be careful to contain the debris
    I've been told. Dreaming`s free.
    Think I'll go, back to sleep.
    Everybody listen, voices in my head
    Everybody listen, do yours say, what mine says?

  7. #7
    Join Date
    20th January 2010 - 14:41
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    Also i forgot to mention often when a throttle preforms "funny" it can be from the routing under the taNk or handlebars
    Have you had the tank off just prior?
    Quote Originally Posted by Katman View Post
    I reminder distinctly .




    Kinky is using a feather. Perverted is using the whole chicken

  8. #8
    Join Date
    23rd August 2021 - 20:52
    Bike
    '81 Honda CB250RS
    Location
    Auckland
    Posts
    9
    I did have the tank off but i checked that the spinny bit of the carb returned home, yet it was still hanging the idle.

    Any preference about where to get helicoil kits?
    Do you reckon I'll get away with leaving the head and motor in the bike, just take the tank and rocker box off?
    I'll put some tape over everywhere that I don't want to get covered in metal.

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