Engine rough, no EGT on 1 or 4 - Now I have something useful to share

[bcg]

Friday, I was in Fairhope, Al AOG with a rough running engine and no EGT on 1 or 4.  Continental had one of their guys take a look but, only had time for a quick fix so I posted looking for another A&P option just in case but, moving the plugs around (not cleaning or anything, just moving) seemed to resolve the issue so, I deleted the post, to the disappointment of some.    I flew a little over 5 hours home and everything seemed fine until this morning when I went to the airport to fly to Dallas.  Fired it up and same thing, very rough and no EGT on 1 or 4 so I taxied over to Dugosh.  We pulled and tested the plugs and they were all good, which I expected since the problem was on the same cylinders and the plugs have only been in for about 10 - 15 hours.  Next I got the high voltage lead tester and tested the plug wires, all good, again expected since they're both new Maggie's harnesses replaced about 10 hours ago.  Borescoped the cylinders and valves were moving fine and looked good, although 4 had fairly heavy lead deposits and and 1 had them less so.  We speculated that maybe the lead was falling into the plugs and causing them to not fire consistently so I cleaned up the plugs, reinstalled and ran the engine.  Initially, it fired up and sounded great.  I planned to taxi into a less trafficked area and do a full power max lean run up to burn some of the lead off the cylinders however, as soon as I started to taxi, it started running rough again and the EGT on 1 and 4 dropped off.

 

At this point, we figure it's got to be the magnetos so I go ahead and pull the right one off and open it up.  Sure enough, the points and rotor have pretty good carbon buildup and there's oil in the mag.  Pulled the left and it's the same except to a lessor extent.  I was going to replace the worn parts except that the repair kit for the right side mag is unavailable anywhere so I instead ordered a couple of overhaul exchange mags and we're going to just swap them out.  If you've never seen the guts of a magneto, here they are.  This is the right mag, the worse of the two, and you can see the buildup.  What you can't see is that when you open the points, it had lots of peaks and valleys and was also pretty black.

 

Anyway, now that we've found the actual cause and it was information I thought might be useful to someone, I thought I'd share the results.  "We moved plugs and it ran ok" didn't seem all that useful to me, which is why I deleted the original post.  As it turns out, it was just coincidence that it "solved" the problem anyway.  Once again, the airplane decided to really break down at home, it's done this to me several times now.  First with the starter, then the alternator while we were en route to home, then the stuck valve that bent a push rod at home and now the magnetos finally not working for good.  I consider myself very lucky that these things haven't happened far from home and I've always been able to just pull the plane into Dugosh when things do break down.  I hope that luck continues, I'm grateful for it.  Sure the breakdowns suck but, I'm running 80+ year old technology in a 61 year old airplane so, those are going to happen.  If they keep happening at home, I've got nothing to complain about.

 

***EDIT TO ADD***

The plane had gotten a little harder to start in recent months, not enough that it concerned me but, enough that I noticed it.  I found that a couple of extra pumps of the throttle with the fuel pump on more or less solved that though, which is why I wasn't too worried about it.  Lesson learned though, engines talk to you, next time something changes I'll dig into it more thoroughly.

[hammdo]

Was gonna ask when the mags were last 500 hr but since it ran fine afterwards, didn’t. This makes more sense!

-Don

[bcg]

4 minutes ago, hammdo said:

Was gonna ask when the mags were last 500 hr but since it ran fine afterwards, didn’t. This makes more sense!

-Don

They were actually inspected March 2022, 352 hours ago.  I would have expected them to be in better shape, not sure why they aren't.  D&B Aircraft Engines did the inspection, based on the logbook entry though I don't know what, if anything, was replaced.  Maybe they just looked, said they were good and sent them back.  It was about 5 months before I bought the plane so, I don't know for sure.

 

I'm happy to have a verifiable cause, I don't like mystery problems that "fix" themselves.

[PT20J]

If D&B is a repair station it will have a work order detailing what was done. You could ask for a copy. 

I would want to know if they replaced the condensers. Metal transfer across the points can caused by a bad cap as I recall. 

[bcg]

If D&B is a repair station it will have a work order detailing what was done. You could ask for a copy. 
I would want to know if they replaced the condensers. Metal transfer across the points can caused by a bad cap as I recall. 

We talked about the cap possibly being an issue, the guys didn't feel like the damage was bad enough for that to be the cause.

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[MikeOH]

46 minutes ago, bcg said:

We talked about the cap possibly being an issue, the guys didn't feel like the damage was bad enough for that to be the cause.

Sent from my Pixel 9 Pro XL using Tapatalk
 

Hmm, I find that a strange argument; a capacitor may not fail completely.

[MooneyMitch]

My opinion, based on previous experience with 500 hour mag checks, I’d have shop replace capacitors during the process. 

[bcg]

17 minutes ago, MikeOH said:

Hmm, I find that a strange argument; a capacitor may not fail completely.

Didn't say it was impossible, just that the wear wasn't consistent with what they've seen when one fails.  It was an educated guess.  We were going to test them but, once we pulled the rotor out, there really wasn't much point.  The repair kits all had new caps in them, if they were in stock, and I'm getting completely different mags that someone already has on a shelf.  One is a factory reman and the other is an overhaul so either way the caps are being replaced.

[PT20J]

FWIW, I have my mags done at Aircraft Magneto Service. They've been in business for a long time and have a good reputation. 

[bcg]

1 minute ago, PT20J said:

FWIW, I have my mags done at Aircraft Magneto Service. They've been in business for a long time and have a good reputation. 

Thank you for the recommendation.  We got them from a shop in Ok sent overnight for AM delivery.  I have to go to Corpus tomorrow and I'm hoping they show up early enough that I don't have to drive, that'll add 5 or 6 hours to the day.  I know, first world problems...  If I wasn't trying to make that trip in the plane instead of the car, I'd have probably sent these out instead of doing an exchange.

[bcg]

New mags are running great, the plane is even a little faster. It starts easier as well. Timing kind of sucked but, I'm glad they're done.

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[KSMooniac]

I hope your mag vendor's name doesn't start with a Q.   Glad they're working for now.