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First mag check attempt bad, second one good

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Aircraft Systems

During runup in a 172S, checked right mag and noted excessive RPM drop.  Pretty much immediately afterwards, went back to both mags, then back to right, and noted that the drop was within limits.  No throttle setting or mixture change during the switch.  Any explanations for why it seemed to fix itself?

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1 Answers



  1. Dave M on Apr 15, 2016

    Could have been carbon deposits on the plugs. Sometimes improperly burned fuel can muck up the plugs. On many airplanes, a solution to a excessively low mag drop is to run the engine lean, at a higer rpm for a number of minutes. This will usually burn off those deposits. It’s possible that when you went back to both, it was enough to remove whatever deposits there mght have been.

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