Intermittent rough running - broken ignition lead

[blaine beaven]

I gave my mechanic the following write up:

 

“As discussed the issue I am having is intermittent rough running when I select the LEFT magneto. My switch goes OFF-RIGHT-LEFT-BOTH

 

Often this rough running is when I do a mag check at 1000rpm, although on occasion it has happened during run up at 2000rpm.

 

I suspect I have a spark plug that might be failing.”

 

They pulled the engine cowls to check the plugs coming from the left mag, and discovered the following:

 

 

 

So the spark plug lead is broken. They were able to repair it with a repair kit, and then tested the lead and it tested ok.

 

I had them stop looking at that point and cowl it back up. I’m going to test run it tomorrow. I’ve got about 8 hours of flying to do with it this weekend. Because it is an intermittent issue it might be hard to track down.

 

My question is: has anyone had a broken spark plug lead before like what I have in the picture, and if so what symptoms did it give you?

 

Thanks!

 

 

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[carusoam]

Congrats Blaine!

I think you found your culprit…

Do you have an engine monitor?

Rough running on one mag should show exactly what cylinder that broken lead is on…

If the funny display and the broken mag are singing the same tune… you are good to go…

If they are different, you have something else to find…

 

I assume you don’t have an engine monitor….

If you do… let’s get some value out of it! 

Best regards,

-a-

[blaine beaven]

Alas, no engine monitor for me or I would have been able to nail the culprit a lot sooner…


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[carusoam]

Great opportunity to lobby the finance administrator with the data and pics that you have recently acquired! 

If cost is an issue…. Many older engine monitors are getting replaced with new Garmin EISs….   The old monitors show up for sale around here…

Install is the expensive part… there is a discussion that is current today of how to do this yourself if you have the skills…

Best regards,

-a-

[GeeBee]

Sometimes a bad lead can cause arcing on the rotor or cap and even back up into the points capacitor. You won't know until you fly it, so go do that and see how it runs.   

[Hank]

I replaced my ignition harness in '17 or '18, and it failed at annual in March '20. My IA said it was falling apart like yours, except mine had the 90º elbows on it [ordered that way, not ones that I added]. He checked and found several others in his shop with similar problems, called Kelly and they sent someone over, about a 10-minute drive. Turned out it was a manufacturing defect and all of us got new harnesses.

So come July '21, I'm loaded up heavy to take the wife on vacation and can only climb 400 fpm away from the runway, at Vx! Landed, runup was awful [good beforehand], isolated it to one cylinder, cleaned both plugs. Awful runup still, 250+ RPM drop. Swapped plugs with a good cylinder, same same. Turned out one cylinder was cold. So no vacation . . .

IA investigated and had to replace the lead to the top plug in the cold cylinder.

I'm now totally uninterested in anything at all coming from Kelly Aerospace . . . . .

[skykrawler]

So did that clear up the problem?  Looking at it, not sure that would cause a problem.  Aviation spark plugs have enclosed connections to reduce static on the radios.