Slight surging, bad magneto

[Scottsware]

Posted in case this happens to someone else.

I noticed the airspeed was less than usual for a given power setting. I was also feeling an intermittent acceleration/deceleration of airspeed. It was so subtle I wasn’t sure if I was imagining it. I had to run the engine richer than usual to get the smoothest operation. It took awhile but I final noticed that all four EGT's were going up and down 20 or so degrees. I have linked a YouTube showing the slight EGT and RPM changes. I did an inflight mag check. Fine on the right, backfiring on the left. If I ran it rich enough I could almost smooth out the left. Another thing I found out was a perfectly acceptable mag check on the ground at 2000 RPM. After the inflight mag check I tested on the ground and found anything over 2200 RPM and the left mag would hardly keep the engine running and it was backfiring.

The mags had been IRAN 80 hours prior. The issue was a bad capacitor in the left mag. It’s all fixed now.

[youtube] https://www.youtube.com/watch?v=NOfM5Do1-xc[/youtube]

[steve_falconer]

So glad you found the issue early and kept safe. I had a few questions.

How do you normally do your in-flight and ground mag checks?

If you know this info: What was the behavior of your EGT readings on your engine monitor when you switched from one mag to the other during your ground check?

[Scottsware]

Normal ground check is at 2000. I lean just enough to get an EGT indication (flying out of DEN). Even with the bad capacitor drop was 90-100 and rise on all four EGT. It wasn’t until I ran it up to 2200-2300 did the issue appear. On the bad mag all four EGT fluctuated wildly while the engine backfired.

For the inflight check I just moved the key as I would on the ground. I have to say that I might be inclined to first go rich prior to inflight mag check and lean on each mag from now on. The backfiring was pretty dramatic at normal cruise leaning. If I ran it very rich it was only rough with no backfiring.

[steve_falconer]

Thank you for the additional detail! Very helpful.