Mag issues and early signs of failure?

[RickWoodall]

Just wondered if those that are smarter than me can give any input.

Latest engine analysis is showing that when I run on just one mag very lean (stress test)....my number one cylinder has slightly uneven spark.

ADD- "Diagnostics show that it is not misfiring, or not firing...just not as even a firing as analysis software would like"

Last engine analysis showed marginal plug in cyl 1.

After the first note, I removed, cleaned, resistance tested and regapped plug.
This time I completely replaced plug. Will see if this fixes it.

Just wondering if this may be a sign the mag is starting to have issues? Is it possible that a mag (only 500hrs) and looks brand new, could give this type of issue to just one of the plugs? Is this a common or no way failure sign?

Harness etc all looks like new... Just wondered if i am missing something here that I should address or test further?

[Raymo]

I believe the general consensus is that mags should be rebuilt every 500 hrs, though my Cherokee went nearly 900 (until I noticed the time on them after purchasing) before they went. However, the guy that did the rebuild noted that parts had been replaced sometime between new and rebuild so the hours may not have been accurate.

That said, I recently did the same test (lean mag check) and the engine ran rough on the right mag but checked fine on the left or when not leaning. I suspected lead fouling, as you did and the plugs checked fine.

Another mechanic suggested the issue is related to the plugs being oil soaked (on the bottom of the cylinder) and was not surprised by the behavior.

[RickWoodall]

Thanks Ray...

I have thought of replacing the mag, but it looks like new, runs like a champ...and just replacing for no reason didnt seem like the best move.

My question is for those that understand mags well, can a mag issue manifest itself as just marginal juice to only one plug...is that even possible or is that very common? Just trying to see if this is a plug issue for sure and replacing will likely solve or if the mag could be the root issue.

[JonJay]

Check your wiring harness with a suitable high voltage tester before going too far.

[tommylewis]

There is a service bulletin for Slick Mags which dictates a 500 hour check. I recently had this done on a Slick Mag from my RV10. While my mag looked fine and ran great, they did replace an addition gear as well as points and condenser. I had this service done because I did not want a failure while on the road traveling. I am glad I had it done. Your choice as many argue service bulletins are not required for experimentals.

[jrs14855]

It is always helpful to specify exact equipment, in this case Bendix or Slick Mags.
TCM SB643B specifies 100 hour and 500 hour INSPECTIONS.
Overhaul is specified at engine overhaul.

[jrs14855]

I should have been more specific that TCM is the current manufacturer of Bendix mags.
Champion for Slick.

[cajunwings]

Rick: the best reason to do the 500 hr inspection is to find issues with the internal parts before they fail. I would have been tempted to swap the weak plug to another cylinder to see if the problem moved there. If problem still in original location you now know plug was good and time to look elsewhere. Your mags output energy sounds ok but you may have delivery issues. Could be the harness but the next place I would look is the distributor part of the mag. The contacts receive the same spark as the plug and over time they erode and get crusty looking and resistance goes up. That fact that it looks like new on the outside doesn't really mean much.

Don B
A&P-IA
RV 9 Rebuild in Progress

[Aggie78]

You might spend 90 minutes here listening to a great EAA webinar by Mike Busch titled "All about Magnetos":

http://www.eaavideo.org/video.aspx?v=1367499350001

I have learned a LOT listening to his webinars...great gouge on the ones on oil and spark plugs as well.

Rob S.

[9GT]

I had one of my Slick mag's fail at just under 500 hours in flight. There were no noticeable warnings prior to the failure. I R&R with a brand new one, then a couple months later during my condition inspection, sent the other mag out for 500 hour inspection/service.