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Old 02-19-2021, 09:32 AM
Robb Robb is offline
Join Date: Sep 2015
Location: Nevada City Ca
Posts: 230
Default Slick 6351 Magneto Mystery

I have a really strange issue with a new to me IO540-D4A5 engine. It has a lightspeed on the right and a slick on the left. When we did the first conditional inspection it appeared to be timed at 20 degrees retarded. I dont see how it could have even ran at that setting. I pulled the mag and it looked fine but had a weak spring on the impulse mechanism. I sent the mag off to have the 500 hr inspection done. I bench tested it and it was fine and sparked on all cylinders. Someone had set it for right instead of left turn. We changed it to left hand and set it to 20 degrees advanced and it was not happy. There was no indication of an issue in the log books and no mention of a relocated timing mark. It was running fine before the inspection but I did notice the Tempest fine wire plugs electrodes were brittle and broke off when the gap was checked? I guess Ill see when we install the mag how it turns out. Never seen this before and there are multiple notes of the mag timing being checked. It has 575 hrs total time and according to the logs it was replaced at 212 hrs by the original builder but no mention of any problems
Aviat Husky A1B
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Old 02-19-2021, 10:11 AM
Ralph Inkster Ralph Inkster is offline
Join Date: Feb 2008
Location: Calgary, Alberta
Posts: 1,243

Maybe check the timing marks on the flywheel being correct? Maybe possible wrong flywheel on the engine?
built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more
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Old 02-19-2021, 10:30 AM
Robb Robb is offline
Join Date: Sep 2015
Location: Nevada City Ca
Posts: 230

checked the flywheel it is correct. Maybe the impulse portion was stuck retarded? Crazy scenario and I have had multiple very experienced guys look at it.
Aviat Husky A1B
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Old 02-19-2021, 11:29 AM
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rvbuilder2002 rvbuilder2002 is offline
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,248

When checking or setting the timing of magnetos you need to turn the prop past TDC to trip the impulse back to its normal state and then reverse to prop rotation slightly to get back to the timing mark. If this isn’t done you will induce a error by the retard/lag value of the impulse coupled.
When you do this you will hear the impulse coupler trip, confirming it is not stuck.
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

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Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
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Last edited by rvbuilder2002 : 02-19-2021 at 11:31 AM.
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Old 02-19-2021, 12:25 PM
Avanza Avanza is offline
Join Date: Jul 2020
Location: Vastervik Sweden
Posts: 46
Smile timing

When everything else fails, consult the manual.,%20Three.pdf

Good luck
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Old 02-19-2021, 06:02 PM
Robb Robb is offline
Join Date: Sep 2015
Location: Nevada City Ca
Posts: 230

We checked the impulse mag when setting timing.
Aviat Husky A1B
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Old 02-19-2021, 06:18 PM
DRMA DRMA is offline
Join Date: Jan 2016
Location: Sugar Land, TX
Posts: 455

Note that the Lycoming IO-540 D4A5 engine should NOT be using an impulse coupled mag. It should be using a slick start unit instead. May not need that with a light speed ignition, as I don't know much about those. But if you check with Lycoming or Slick, they will both tell you not to install an impulse coupling on this engine.
Dave Macdonald

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Old 02-19-2021, 07:10 PM
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rocketbob rocketbob is offline
Join Date: Jun 2006
Location: 8I3
Posts: 3,669

There is no mystery here. It works or it doesn't.

Verify snap of impulse coupling on TDC #1 by looking thru top plug hole at piston with flashlight. Then check flywheel marks. Back off 35 deg or so advanced, and bump prop to 25 and check timing with buzz box. Timing is 25 degrees advanced, not 20, on a parallel valve 540.

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Old 02-20-2021, 07:38 AM
mcgaughy mcgaughy is offline
Join Date: Mar 2009
Location: Fayetteville, NC
Posts: 57

I have never heard of someone changing rotation direction on a mag. I was always taught to read the data plate on the mag and simply use that info to time, etc .... and of course, having the correct mag for the engine. But this is all relative to certificated aircraft maintenance. Changing rotation changes firing order and I guess some of the impulse parts would have to be changed. And Im also guessing internal timing would have to be reset but idk. Interesting.


Last edited by mcgaughy : 02-20-2021 at 11:27 AM.
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Old 02-20-2021, 08:07 AM
wilddog wilddog is offline
Join Date: Feb 2008
Location: va.
Posts: 617

Can the impulse mechanism be removed and the same mag put back on the engine and operated as a non impulse without further modifications? I have SDS on the right and can start with only it. Slick shows different p/n for the rotor and case of the impulse and non impulse mags.
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