Ask Paul: What is causing mag drop?

Q: I work on a IO-360-MIA Lycoming engine. My problem is that the right mag has a drop of 220 rpm. I have done internal timings, engine to mag timings, ignition leads check and cleaned the plugs. I have done nothing to the left mag, and now the left mag is dropping to 210 rpm.


A: Interesting question and hopefully somewhat easy to troubleshoot Angi Lisa. First, I have to ask a question to answer your question: What happens if you manually lean the mixture while doing a mag check? It may be just a slightly rich mixture causing the higher than normal drop-off.

Just so we’re all on the same page, I hope you are checking the magnetos with the engine at normal engine operating temperatures. For a proper magneto drop-off check on your engine, the rpm should be between 1800 and 2000 rpm.

Don’t overlook the possibility that the air filter may be dirty, reducing the intake air volume, which results in a richer mixture.

Even though you didn’t mention it, I’m curious to know what the fuel flow is at full throttle at takeoff power. I’m not certain if a partially plugged fuel nozzle could cause this problem, but if the fuel flow gauge needle goes to the top of the gauge, this could indicate a plugged nozzle.

I think you’ve done all of the things with regard to the mags and ignition harness, so let’s hope the above suggestions will put you on the right path to solve this situation.


  1. Ron S. says

    I also had the problem with arcing at the spring insulator at the top of the plugs on a Lycoming 0-320. It only takes a little corosion and contamination to start conducting there. After a through cleaning I wrap a little teflon tape around the top of the wire insulator which helps keep it clean.

  2. John says

    Check the capacitor inside the magneto – the mounting bracket has a habit of breaking off causing the case to lose the earth return

  3. JimH. says

    I agree that the Champion plug could have the internal resistor value greater than 2,000 ohms and will cause poor engine performance. It’s a well documented issue, vs Tempest plugs.
    The other possibility is the internal capacitor in the mags. We’ve had two fail in about 3,000 hrs TT on the mags in a C172R. Also, make sure the breaker point gap is correct and that the points are clean- no oil inside the mag.

  4. Rich says

    I chased a higher than desired mag drop a short time ago on my left mag and after cleaning all plugs and swapping them left to right, mag drop stayed the same.
    I checked all; the leads with a high voltage tester and all seemed ok.
    Finally after looking closely at the springs that go in the top of the spark plug Isaw some discoloration where the end of the spring mates with the contact inside the plug.

    I cleaned the end of the springs and voila!! Problem gone.

    I had this same thing happen about 20 years ago on this very same engine.
    I got my memory refreshed.

  5. Richard says

    I’m curious and hope there will be a follow up as to whether the suggestions cured the problem.

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