O-470U suddenly idling 150 RPM high

Discussion in 'Maintenance Bay' started by George Mohr, Sep 20, 2021.

  1. George Mohr

    George Mohr Line Up and Wait

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    Seemingly overnight. No other indications of any problems. Thoughts?

    Thanks!
    G
     
  2. Dan Thomas

    Dan Thomas Final Approach

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    1. Sticky tachometer. Needs calibrating, maybe replacing. The oil in the tiny bearing in it between the input shaft and the indicator needle gets dired out and sticky and the needle gets dragged to a higher reading.

    2. Creeping idle mixture needle, creeping from a much-too-rich position to start with. Not likely.

    3. Leaking induction manifold.

    4. Loose, shifted magneto. Less likely.

    5. MOST LIKELY: the throttle cable housing clamp has let the housing slip and the throttle control is no longer closing the throttle to the idle stop screw. Or the rod end on the control is loose and the wire shaft is backing out of it. Or the lever on the carb is loose.

    Lots of possibilities. You need a mechanic to chase it.
     
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  3. George Mohr

    George Mohr Line Up and Wait

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    Thanks,

    1) EDM 900, so unlikely
    2) Will do the mixture pull / raising RPM test next time im at the airport
    3) All the EGT's look pretty normal, so hoping its not this
    4) Same, as above, in flight everything is spot on. I doubt it's timing
    5) So yeah, I agree its something with the linkage.

    TY!!
    G
     
  4. Clip4

    Clip4 Final Approach

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    My vote is a induction leak.
     
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  5. Stewartb

    Stewartb Final Approach

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    I’d check the throttle arm and mixture arms very closely. It those are good I’d check the intake rubbers at the Y pipe. Leaks aren’t easy to spot. If your manifold pressure wandered a little on the last flight or two? Induction leak.
     
  6. George Mohr

    George Mohr Line Up and Wait

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    And the winner is..... induction leak! Two of the hose clamps on the silicone couplers in the intake manifold were very loose. Tightened and all is good. TY for the help, folks.
     
  7. Dan Thomas

    Dan Thomas Final Approach

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    A 150 RPM rise indicates that the idle mix is likely set too rich. Consult the airframe manufacturer's service manual for the correct setting.