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It’s been a busy week. One week ago I had to cancel a flight to Carp, for the EAA Chapter 245 Fly-In Breakfast, due to a hangar door opening winch failure. We quickly determined that the reduction gear box had failed, but the gear box had no data plate or markings on it, so it was not possible to source a direct replacement. We found a larger gear box at Princess Auto, but it wouldn’t fit on the existing mount, so quite a bit of barnyard engineering was required to get it all working again. We (those with aircraft trapped in the hangar) had several work sessions over the course of the week, and finally got it all sorted out on Wednesday evening.

Terry and got up early yesterday morning, to fly to Green Bay, WI. But that plan fell apart at the engine run-up prior to take-off. Terry had noted that she thought the engine sounded a bit different than normal when we were taxiing, but I couldn’t hear anything strange. But, when I advanced the throttle for the run-up, I could tell that the engine didn’t sound right, and the amount of rpm achieved for a given throttle position was quite a bit lower than normal. When I selected the magneto OFF, the rpm increased by 300 rpm, which was very abnormal. This clearly indicated that the magneto timing was much too far advanced.

I left the mag off, and taxied back to the hangar. I removed the cowling and hooked up the mag timing box. I heard a momentary nasty noise from the back of the engine as I turned the prop to Top Dead Centre. That noise didn’t come back, but there was no indication that the mag points were opening and closing at all. We clearly weren’t going to Green Bay, so we unpacked and headed home.

Today I went out to the airport early to dig into the issue. I found that the mag base was broken, and the impulse coupling was very loose. There were signs that the impulse coupling had been hitting the base of the mag. The teeth on the magneto drive gear looked in perfect shape, and I couldn’t see any damage on the teeth on the drive gear in the accessory gear box.


I thought at first that some of the small mag case pieces were inside the engine, but looked around on the floor under the back end of the engine and found that all the missing pieces were accounted for. Whew!


The aircraft currently has one magneto and one Lightspeed Plasma II electronic ignition. The Lightspeed ignition works well, providing more ignition advance than the magneto at cruise power settings for increased efficiency, but it does require aircraft electrical power to function. The magneto will function even if the aircraft electrical system has completely failed, which is a huge plus.

I’ve been watching the developments at E-Mag for several years. They developed two closely related electronic ignition systems that fit in the magneto spot on the accessory case. The E-Mag, like the Lightspeed, required aircraft power. Its brother, the E-Mag model P (aka P-Mag) has an internal alternator, which generates power sufficient at 800 rpm to run the ignition. The P-Mag requires aircraft power to get the engine started, but after the engine is running it will function even if the aircraft electrical system fails, as long as the rpm is kept above 800 rpm. The first design P-Mags had some significant in-service problems, but they updated the design to address the problems, and the latest configuration (series 114) has proven itself in service.

I had decided to replace the mag with a P-Mag when the mag was due for its big inspection at 500 hours. Now that it has failed, I’ll do that change now. I’ve spent several hours planning the mod, making a list of all the things I should require, and I’ll order all the goodies Monday morning.

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  • Today I ordered the P-Mag, drive gear, gaskets, plug
    adapters, plug wires, spark plugs, wire, wire terminals,
    circuit breaker, etc that I think I need to install the P-Mag.
    In theory, I should have it all in my hands by Thursday
    evening, and will be beavering away at this mod over the

    Kevin Horton

  • Hi Kevin,

    When I was last at SNF, the fellow at the EMag kiosk
    suggested that for lighter props such as the MT, the mag
    timing should be set 1 deg later than normal to decrease
    chances of prop swinging backwards (or something like
    that, I do not recall exactly) on start up. You may want to
    confirm this with them.


  • Hi Normand,

    Thanks for the info.

    I was aware of this, from the P-Mag Installation and Operation Manual. I intend to set the timing with the engine 3 deg after TDC. This will retard the timing curve 3 deg from P-Mag's nominal values, but that is good in my opinion, as their nominal values have it 3 deg further advanced at high power than is recommended by Lycoming, which makes me a bit uncomfortable.

    Kevin Horton

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