AZOutlaw Posted October 30, 2025 Author Report Posted October 30, 2025 Update: Bottom line, the problem still exists. We checked the timing and did think it was off a little so we adjusted that. Fired it up and first check had the same result. (-150 Left and -40 Right) We shut it down, discussed a few things and decided to try again. This time it was perfect, about a -35 mag drop on each side, or what it always did in the past. Took it up and all seemed good. In flight mag checks were equal and uneventful. Landed and did a check before shut down and got a -65 on the left. Went back a day later and got the same (-150 Left and -40 Right). Borrowed a different timing box, bumped them slightly and felt they were perfect, same result and this time the inflight test was bad on the left, like it was a week ago. Swapped bottom and top plugs thinking the issue might move to the other mag. It did not. Same result, but what is interesting is I did the second start up 30 seconds after shutting down as before, and got perfect mag drops again. This got me thinking heat and resistance was an issue. Pulled all the plugs and did a resistance test, twice, and got these readings. Seems that 5B is reading low and 3B is high but all the rest are really close. I found this online. "Champion Spark Plug Company’s Engineering Manual gives the nominal value for their resistor type of aviation spark plugs as 1000 - 1500 ohms" Anyone have real world experience on what is good and bad readings for a Champion plug?
Fly Boomer Posted October 30, 2025 Report Posted October 30, 2025 1 hour ago, AZOutlaw said: Anyone have real world experience on what is good and bad readings for a Champion plug? I think those look good. You know a Champion plug is not so good when the resistance gets well up into the thousands (5,000).
Fix Posted October 30, 2025 Report Posted October 30, 2025 Can you Swap the Mags from Left to Right and vice versa? If the problems moves you know it's the mag... 1
Hank Posted October 30, 2025 Report Posted October 30, 2025 When my C had uneven mag drop, I could burn off and do another run up and be good. Over several flights, the uneven drop got worse, then wouldn't burn off. Turns out one of the leads in my 2-year-old ignition harness was bad. Since replacement, everything has been fine.
EricJ Posted October 30, 2025 Report Posted October 30, 2025 +1 that it could be the harness. Also, +1 to learn how to burn off fouling. That'll save a lot of hassle dealing with ignition issues and also help to understand whether it is just repeated fouling or an actual component failure. An additional thing that might be looked at is the condenser on the funky side, but that may be a last resort just short of sending the mag off. 1
PT20J Posted October 30, 2025 Report Posted October 30, 2025 The plug resistances look fine. If it were mine, I'd test the harness with a high voltage ignition cable tester. If your mechanic doesn't have one, Aircraft Tool Supply's E5A is only $150.
Jackk Posted October 30, 2025 Report Posted October 30, 2025 Which side has the impulse coupling normally that would only make like a 50rpm ish diff, but still
AZOutlaw Posted October 31, 2025 Author Report Posted October 31, 2025 Thanks guys, just a refresher. It's a brand new harness two weeks ago and it was tested, for good measure, last week. Mags were rebuilt this summer and only have 6 hours on them. I'm very familiar with burning off fouling and have done that numerous times. Timing has been checked and re-checked. Fuel injectors were removed and cleaned. I'm leaning towards the fact that my Magneto rebuild didn't get rebuilt properly. We pulled them out and sent them in when this started after first replacing the spark plugs. The problem was there before the rebuild and remains still. 1
PT20J Posted October 31, 2025 Report Posted October 31, 2025 11 minutes ago, AZOutlaw said: Thanks guys, just a refresher. It's a brand new harness two weeks ago and it was tested, for good measure, last week. Mags were rebuilt this summer and only have 6 hours on them. I'm very familiar with burning off fouling and have done that numerous times. Timing has been checked and re-checked. Fuel injectors were removed and cleaned. I'm leaning towards the fact that my Magneto rebuild didn't get rebuilt properly. We pulled them out and sent them in when this started after first replacing the spark plugs. The problem was there before the rebuild and remains still. Good bet. That’s about all that’s left. Not sure what was done: was it an IRAN or an overhaul? I’d consider sending the left mag to Aircraft Magneto Service.
takair Posted October 31, 2025 Report Posted October 31, 2025 Getting into the “long shot” realm, check for exhaust leak onto a plug. Also, confirm ignition switch working correctly, at idle go off and make sure it cuts off. Check at different RPM? Does it behave the same? Magneto, as you already suggested, would not be the first bad overhaul in the world. 1
CCAS Posted October 31, 2025 Report Posted October 31, 2025 7 hours ago, AZOutlaw said: Bottom line, the problem still exists. Any chance there's an issue on the accessory case side of the equation (drive coupling bushing/retainer, magneto drive gear, idler gear assembly)? Those might manifest with intermittent results. Couple other questions mostly out of curiosity: what RPM are you starting the mag checks at? when you perform the mag check does the 150 RPM drop occur pretty rapidly or is it a slow steady decay? what kind of density altitudes are you seeing when you run the checks (do you perform the mag checks Full Rich or leaned for higher DA)?
EricJ Posted October 31, 2025 Report Posted October 31, 2025 3 hours ago, AZOutlaw said: Thanks guys, just a refresher. It's a brand new harness two weeks ago and it was tested, for good measure, last week. Mags were rebuilt this summer and only have 6 hours on them. I'm very familiar with burning off fouling and have done that numerous times. Timing has been checked and re-checked. Fuel injectors were removed and cleaned. I'm leaning towards the fact that my Magneto rebuild didn't get rebuilt properly. We pulled them out and sent them in when this started after first replacing the spark plugs. The problem was there before the rebuild and remains still. Did Bruce in Camp Verde rebuild the mags by any chance? If so, he's a really good resource for debug of unusual ignition issues. He got me through some oddball stuff before. In my experience he's very good about supporting his work.
AZOutlaw Posted November 1, 2025 Author Report Posted November 1, 2025 On 10/30/2025 at 8:46 PM, EricJ said: Did Bruce in Camp Verde rebuild the mags by any chance? If so, he's a really good resource for debug of unusual ignition issues. He got me through some oddball stuff before. In my experience he's very good about supporting his work. Yes it was Bruce and I agree, he's great! I took the left mag to him yesterday and he tested it out. It appears we got a bad capacitor. He put it on a tester and we could see intermittent sparks. Not a lot but you could sure see a few drop out. He found an old capacitor in a drawer and put that on and it cleared things up. He didn't have a new part to match so I'll get it back next week and update the results. 3
EricJ Posted November 1, 2025 Report Posted November 1, 2025 2 hours ago, AZOutlaw said: Yes it was Bruce and I agree, he's great! I took the left mag to him yesterday and he tested it out. It appears we got a bad capacitor. He put it on a tester and we could see intermittent sparks. Not a lot but you could sure see a few drop out. He found an old capacitor in a drawer and put that on and it cleared things up. He didn't have a new part to match so I'll get it back next week and update the results. Nice! The condenser was one of my guesses above. Regardless, glad that got sorted out. 1
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