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Thread: The 50 RPM Increase at idle cutoff

  1. #1
    Pacerfgoe's Avatar
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    Default The 50 RPM Increase at idle cutoff

    OK, so we all know that if everything is set up correctly you should see a 50 RPM increase in idle speed as you slowly go to mixture cutoff.

    I was trying to get this to happen yesterday without success, burned the $hit out of my arm doing the adjustments and gave up.
    I'm thinking I wasn't doing it correctly....I idled it at 700 rpm and slowly went to cut off....what RPM are you supposed to start at when doing this procedure, and how slow or fast should one pull the mixture to cutoff?

    BTW....it's a new engine installation, new cylinders, freshly gone through carb, and primer doesn't leak.

    Your help is appreciated.
    Forrest Miller

  2. #2
    andya's Avatar
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    Default Re: The 50 RPM Increase at idle cutoff

    I seem to recall that the rpm to do that check was 1000 rpm.
    Cannot recall the document or article that calls that out.
    I'm sure someone will pop in with the reference.
    When you pull the mixture back, you should pull it very slow to notice the rise in RPM as you get close to cutoff
    "Progress is our most important problem"

  3. #3

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    Default Re: The 50 RPM Increase at idle cutoff

    Heres what I have saved for the MA4-SPA
    20921DFB-ECB3-49D4-8D0E-831D42DC0172.jpeg
    Attached Images Attached Images

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    andya's Avatar
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    Default Re: The 50 RPM Increase at idle cutoff

    thanks JV obviously had that information wrong.
    "Progress is our most important problem"

  5. #5

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    Default Re: The 50 RPM Increase at idle cutoff

    Don’t “slowly” go to idle cut off, go from full rich at idle speed to full idle cut off in one smooth brisk motion while observing the RPM. That’s where you should see the 25 to 50 RPM rise just before it shuts off.


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  6. #6
    Pacerfgoe's Avatar
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    Default Re: The 50 RPM Increase at idle cutoff

    Thanks for the replies guys, next time I'm out at my plane I'll try your recommendations.
    Forrest Miller

  7. #7
    Pacer42Z's Avatar
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    Default Re: The 50 RPM Increase at idle cutoff

    To lean the idle mixture, do I need to turn the screw clockwise (in)?

    Juergen
    Pacer N 3342Z

  8. #8
    cozzmo81's Avatar
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    Default Re: The 50 RPM Increase at idle cutoff

    The instructions for idle mixture adjustment are found in the lycoming operators manual. Page 5-5

    The mixture adjustment screw should be marked with an L and R with corresponding arrows as to which way to turn.

    Here are the instructions from the lycoming manual.

    e. Idle Speed and Mixture Adjustment.
    (1) Start the engine and warm up in the usual manner until oil and cylinder head temperatures are normal.
    (2) Check magnetos. If the “mag-drop” is normal, proceed with idle adjustment.
    (3) Set throttle stop screw so that the engine idles at the airframe manufacturer’s recommended idling RPM. If the RPM changes appreciably after making idle mixture adjustment during the succeeding steps, readjust the idle speed to the desired RPM.
    (4) When the idling speed has been stabilized, move the cockpit mixture control lever with a smooth, steady pull toward the “Idle Cut-Off” position and observe the tachometer for any change during the leaning process. Caution must be exercised to return the mixture control to the “Full Rich” position before the RPM can drop to a point where the engine cuts out. An increase of more than 50 RPM while “leaning out” indicates an excessively rich idle mixture. An immediate decrease in RPM (if not preceded by a momentary increase) indicates the idle mixture is too lean.
    If step (4) indicates that the idle adjustment is too rich or too lean, turn the idle mixture adjustment in direction required for correction, and check this new position by repeating the above procedure. Make additional adjustments as necessary until a check results in a momentary pick-up of approximately 50 RPM. Each time the adjustment is changed, the engine should be run up to 2000 RPM to clean the engine before proceeding with the RPM check. Make final adjustment of the idle speed adjustment to obtain the desired idling RPM with closed throttle. The above method aims at a setting that will obtain maximum RPM with minimum manifold pressure. In case the setting does not remain stable, check the idle linkage; any looseness in this linkage would cause erratic idling. In all cases, allowance should be made for the effect of weather conditions and field altitude upon idling adjustment.


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    Adam Rhoads

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