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Request help with mag problem

koupster

Well Known Member
Yesterday, my left Slick mag failed an inflight mag check forcing a turnback on a Pilots N Paws mission. This is an impulse coupled mag.

Runup at 1800 rpm on the ground showed no problems.
During an inflight mag check, the engine would "hiccup" every so often. At first, I thought it was a jolt from turbulence, but the frequency increased to more than once a minute. It seemed to be a split second complete interruption of power, rather that a misfire on one or more cylinders. The power interruption was so brief that no change in EGT or airspeed was noticed. Changes of the mixture lever position, switching tanks, and turning on the boost pump had no effect.

Plugs are nice and clean (properly gapped, also) and resistances check OK (two around 1300 ohms and two around 1900 ohms). The ignition cable tester showed a spark in the window, but every so often, the spark skipped a beat and then returned. Same result on all four plug leads. This is my first time using a cable tester which was bought new. I only have a half page of instructions that I downloaded. If anyone has experience with cable testers, please chime in or contact me via email.

Am borrowing a set of plugs to test with today. It's a very long shot, and I expect no change, but it's way easier than pulling the mag. After that, I'll try disconnecting the p-lead to investigate a possible switch (standard keyed Off, L, R, Start switch) problem. I'll be careful with the hot mag.

If no solution after this, I'll be pulling the mag, and trying to find someone local that can test the mag at high rpm. If that fails,I'll be sending it out.

Any hints, advice, personal experience, or suggestions will be welcomed. I'd rather not pull the mag if it's not necessary.

Thanks.
 
Carb or Injection?

Check the p-lead wiring that there isn't rubbing / short say on the firewall pass-thru and elsewhere.
 
Carb O-360

Will check wiring, and eventually check with p-lead disconnected if nothing else turns up.

Thanks.
 
When you use your cable tester make sure you have all leads disconnected from all spark plugs and away from them.
 
You say it failed an in-flight mag test - are you saying that you ran on one mag for an extended period, and that is when you noticed the hiccups? What altitude and RPM's were you running?
which mag is it, and do you have one mag to the top plus and one to the bottom plugs?
 
I switched to the left mag for an inflight mag check. Then waited a bit before leaning to let the EGT readings stabilize. Got the first hiccup, and didn't touch anything for a bit. The hiccups reoccurred at a rate that seemed to get more frequent. Then switched tanks, turned on the boost pump, and varied the mixture with no change. Switched back to "Both", and the engine ran normally.

Approximately 8000' MSL and 2400 rpm when the hiccups occurred.

"which mag is it?" Left mag (with impulse coupler). It runs the bottom plugs. Right side is a Lightspeed running the top plugs.

Failed to mention that I did a full power runup after landing on just the left mag. Engine ran smoothly.

Is it possible that this is a heat related problem affecting mag internals such as the coil or capacitor?
 
Is it possible that this is a heat related problem affecting mag internals such as the coil or capacitor?

Possibly. I just replaced the left mag on a O-320 due to heat related failure. Mags drops were fine during run ups and there were no issues in cruise, but hot starts rapidly worsened until it wouldn’t start at all when hot. Replaced the slick with a Surefly and it now starts like a car hot or cold. And runs smoother, better, etc.
 
I had same thing happen.

I had the same thing happen, after flying it for 3-4 hrs with a hiccup every couple minutes, it got way worse, backfiring running real rough. I had a mechanic pull the one mag, a gear had worn enough to allow the timing to jump to another post on mag. He replaced the gear and it ran fine. I checked and I was over tbo on the mags. I sent both to be rebuilt in Montana and now it runs great. My suggestion is to have the mag inspected by an overhaul shop. I’m thinking you have some internal parts with excessive wear.
 
Magneto Hiccuping

The points are most likely the problem. After only 29 hrs in service a mag I examined had the same symptoms. After tear down one of the contacts on the lever arm of the points had become dislodged and was bouncing around. It was a manufacturing defect.
 
I switched to the left mag for an inflight mag check. Then waited a bit before leaning to let the EGT readings stabilize. Got the first hiccup, and didn't touch anything for a bit. The hiccups reoccurred at a rate that seemed to get more frequent. Then switched tanks, turned on the boost pump, and varied the mixture with no change. Switched back to "Both", and the engine ran normally.

Approximately 8000' MSL and 2400 rpm when the hiccups occurred.

"which mag is it?" Left mag (with impulse coupler). It runs the bottom plugs. Right side is a Lightspeed running the top plugs.

Failed to mention that I did a full power runup after landing on just the left mag. Engine ran smoothly.

Is it possible that this is a heat related problem affecting mag internals such as the coil or capacitor?

It could be. Sometimes when coils go bad, they only show the failure once the composite shell expands from heat. The long copper wires are encapsulated in that shell. The fact that it is cycling good/bad kind of points away from this as an issue.

It could also be the e gap or several other things. E gap is a critical setting, as the magnet can only produce enough energy to spark within a very small range and point wear changes it. If you are right on the fringe, the no spark problem can come and go at will and higher RPMS will be more problematic, as the dwell time decreases.
 
Just curious, what prompted you to perform an inflight mag check?

The OPs post is a reason in itself to do them. In flight mag checks will sometimes show problems that don't manifest in ground tests. Both the additional RPM and the altitude can intensify problems. Thinner air makes it harder for the spark to jump the gaps in both the cap and the plug.

Larry
 
I had a similar thing happen about 40 hours ago; my left mag which had less than 40 hrs since last being serviced. Nearly new harness, and the plugs checked out okay.

My 'solution' was to replace it with a P-mag, problem solved.

Okay I know that's not really an answer to the OPs question but there you go. I had been intending to make the switch some time anyway, so this just seemed like time. This was the left mag, the right one was already running a P-mag. I was surprised that I could really tell the difference with two Pmags vs one - it's definitely smoother especially at idle. And it will save money on spark plugs over time :rolleyes:
 
Thinner air makes it harder for the spark to jump the gaps in both the cap and the plug.

Larry

I think you got it backwards. Thinner air makes it easier for the spark to jump gaps, sometimes to the wrong place inside the mag, leading to high altitude mis-fire. Some turbo’d (high altitude) engines come with pressurized mags (mostly to sweep ionized particles out).
 
I think you got it backwards. Thinner air makes it easier for the spark to jump gaps, sometimes to the wrong place inside the mag, leading to high altitude mis-fire. Some turbo’d (high altitude) engines come with pressurized mags (mostly to sweep ionized particles out).

Interesting. I had read that the pressurized mags were in place to increase the ambient pressures in the mag internals in order to improve sparking ability that was degraded in thinner air. I have no scientific data, so will defer to your explanation as it makes sense; Easing the ability to jump gaps makes carbon traces a severe issue.

Larry
 
Older military radios were pressurized with a bicycle pump to prevent internal shorting as current jumped gaps with reduced pressure during climbout. You could tell the radio had lost pressure as it’s ability to transmit and receive got worse as you climbed.
 
light bulbs going off now. I assume this is why vacuum tubes were created before silicone chips came into being.
 
Here is the “Paschen curve” that shows how far a spark might jump in air. At say 20,000 volts a spark can jump 3 times farther at 10,000 feet than sea level. Interestingly this is only the case for atmospheric pressures. as you go higher and the curve reverses in in a vacuum the distance the spark will travel decreases.
 

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Did you change the offending plug yet?

I recently had a crack in the ceramic on the lead side of the plug that was shorting out the dancing pixies. Ohm check on plug doesn't reveal this because most multimeters don't output much voltage for resistance checks. This is exact symptom I had - normal run-up, no firing at altitudes >6000'. I found mine by noticing the EGT running noticeably higher that others. In-flight mag check confirmed it wasn't firing.

https://vansairforce.net/community/showthread.php?t=204148
 
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Did you change the offending plug yet?]

Tim,

Thanks for your response. In my case, it wasn't one plug that stopped firing. The engine would quit for a fraction of a second when running only on the left mag and then come back to life.

I've replaced all four plugs that the left mag fires on the slim chance that a bad plug could have triggered an intermittent mag failure. I know it's a slim chance, but it seemed worth doing before pulling the mag. Haven't had a chance to do a test flight yet.
 
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