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  1. #1
    Toothy's Avatar
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    What causes the idle to race as the engine gets hot?

    It runs great with good throttle response until it warms up and then it'll race. I can get it to knock down at a stop by letting the clutch out to where it engages and then it brings down the idle to normal.


    Ive got stock pipes and jetting, knockoff pod filters, and 1and a half turns out on the main pilot screw.

    ?


    Thanks, Dave.

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  3. #2
    Supporting Member jensen's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    Hi,


    leaking air on intake or/and out of sync or/and airscrew,

    Jensen
    assembly of Japanese motorcycles requires great peace of mind (Zen and the Art of Motorcycle Maintenance / Robert Pirsig)

  4. #3
    Toothy's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    I haven't yet synced the carbs and I don't know what an airscrew is.

    The pod filters seem to allow very generous airflow.

    Is what you're suggesting as a remedy a more restrictive filter situation will prevent the idle from racing as the engine heats up?

  5. #4
    Senior Member asdfqwer426's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    Quote Originally Posted by Toothy
    I haven't yet synced the carbs and I don't know what an airscrew is.

    The pod filters seem to allow very generous airflow.

    Is what you're suggesting as a remedy a more restrictive filter situation will prevent the idle from racing as the engine heats up?

    I believe what he means is an air leak between the carburetor and the engine. either a bad gasket between the engine and carb boot, or the rubber carb boot is too hardened and the clamp isn't making an air tight seal. i know an air leak here would cause fast idling, i think too much filter flow will just cause mixture issues, not the racing idle.

    the air screw is the mixture screw that you have out however many turns, there are many names for this and can get confusing.

    the pod filters might be part of the issue, but do some plug chop tests to really see whats going on. it seems annoying, having to buy all these plugs to do a bunch of tests over and over, but it really is very helpful and the only way to figure it out. I had the same issue, i thought the filters were flowing too much, causing lean running. after plug chops, i found out they were too restrictive actually and i had to go down in get size.

    to start i would double check the mixture screw is the same turns out, and sync your carbs. then look for leaks and do some plug chops.
    Nick

    CL350 K4 (pamco installed)
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  6. #5
    Toothy's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    By plug chops, do you mean removing the spark plug to inspect the coloration?

  7. #6
    Super Moderator nigelrharris03's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    Plug chops "mean what they say on the tin". You cut the engine whilst running then remove the plug and cut off the screw thread and see what colour the insulator is right down to its base.
    Please feel free to correct me if I have misunderstood this procedure I wont feel too embarrased.
    Nige
    CB450K0 Bomber 1967
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  8. #7
    Super Moderator J-T's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    Step 1. Acquire a six-pack of your favorite beverage.

    Step 2. Google the term "plug chop" and start reading and/or watching videos.
    JT
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  9. #8
    Senior Member delineator's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    hanging idle can also be caused by weak spark advance springs, so when you rev up, they stretch out, and are meant to pull in the advance as you let off the throttle. if they are weak, the timing will stay advanced and the idle will hang.

    easiest way to check this is with a timing light. if the timing sits at the marks at idle, then advances to between the 2 "advanced" marks, then when you let off the throttle, the timing stays at the advanced marks, this is your problem. if the timing returns back to the idle marks, but the rpms still hang, then this is not your problem.

    if this is the problem, the solution is to try and find replacement springs of some kind, or bend in the hooks of the existing springs a bit to "tighten" them up. while your in there, clean up the advancer unit really well so it rotates very easily.
    1971 CB450 (K4) - industrial cafe
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  10. #9
    1970Cafe350's Avatar
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    Re: What causes the idle to race as the engine gets hot?

    "I believe what he means is an air leak between the carburetor and the engine. either a bad gasket between the engine and carb boot, or the rubber carb boot is too hardened and the clamp isn't making an air tight seal. i know an air leak here would cause fast idling, i think too much filter flow will just cause mixture issues, not the racing idle"

    An easy way to test this, is to spray some carb cleaner at joint where the carb/rubber/ clamp meet, if it gets sucked in and the idle raises, then you have an air leak.
    70 Cafe 350

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