Avionics Master Switch / Mag Issue

[isaacpr7]

Just re-phrasing by post above: Should have been a question more like: If both are grounded, how would it sustain firing?

Yves

That's because the SOS is what lights up the plugs through the left mag contact brakers. When you release the ignition switch and goes back to both, then both mags are actively providing the spark for the plugs at 25deg BTDC.

[isaacpr7]

Interesting and timely discussion. First engine run today out of annual. Had a very difficult time starting engine (67 C and it usually starts easily in a couple of turns hot or cold). Finally got engine running and shortly thereafter discovered left mag was completely dead. Back to my hanger, cowl off, checked p-lead and it was a bit loose. Removed, clean, and installed again. Engine started without much difficulty this time and the left mag was alive again. Run up was absolutely normal and subsequent test flight went very well. Honestly, I don't know what got fixed other than the p-lead being slightly loose which somehow grounded the left mag? Anyway, to the poster, your symptoms were same as mine. Nearly ran the battery down trying to start, but the P-lead appeared to be the culprit in my case

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Hector,

 

What happened with yours is that the left mag was grounded due to the loose P-lead.  The left mag as a spring with a tab that is pushed away from the mag case when the p-lead is installed. If the p-lead becomes loose, then the tab contacts the case and grounds the mag. This is why the left mag has a specified torque for the nuts and the right mag is only finger tight. The tab is a safety feature built in to our type of mag so that you cannot crank the engine with the p-leads removed; however, it still does not help if the p-lead wire is cut because the tip is still pressing the tab.

 

My newest issue is that I now get spark but not without detonation. Another mooniac suggested that the engine might be timed on the exhaust stroke and that makes more sense to me. I will have my mx check that out for me this week.

[isaacpr7]

Below is a copy of an email sent to me by the mag shop, further explaining the SOS operation:

 

Isaac,

Sorry for the delay ion this response. We have family in town and I was away from my phone.

 

The SOS vibrator is not a timed event. It puts out a continuous "shower of sparks" that is controlled by the opening and closing of the retard breaker or contact set. An easy way to check this retard timing is to position the engine at #1 cylinder TDC, back up the prop to 30 degrees BTC (assuming 25 degrees is normal timing for that engine), and remove the starter wire from the starter motor. Then, remove one ignition lead from the #1 cylinder coming from the left magneto and hold it in close proximity to a ground. Have someone place the mag switch in the start position with the master on. This will activate the SOS while grounding the left and right main breakers. As you advance the prop to TDC, you should get a spark from the removed plug wire at the correct position relative to TDC.

 

Your retard magneto is set to open 25 degrees after the main set of contacts opens. This places that retard ignition approximately at TDC. A few degrees one way or the other will not be a problem.

 

If the engine is backfiring through the exhaust, it usually means too much fuel is being introduced during start sequence. If the retard contacts were timed to open too early, the engine would kick back during the start. If they are timed to late (meaning past TDC), the engine would still start and the back firing would be rare.

 

Check this out and let me know what you find.

 

Regards,

Paul

[Alhall]

I like your post from the mag shop on how to test the retard timing. One thing I noticed when do this test it should be on the compression stroke for the number one cylinder (front right cylinder) not the exhaust stroke.

Al

[rockydoc]

And I thought Mexican mx weren't as sharp as USA mx.  Did this article ever give me cause to revisit that opinion.  An AP that doesn't understand mags--let alone basic continuity testing of wires--Wow!  Now that's embarassing.  And, I imagine he's charging around $100 USD/hour vs the $80USD/day rate in Mexico.  Jeees Luis

[N601RX]

The other thing I would add to what the mag shop said is to remove all the top plugs. You don't want it to accidentally start up.

[isaacpr7]

My fellow Mooniacs,

 

My baby is finally all fired up!!!  We rechecked that the timing and it was on the compression stroke when verified with the buzzer.  After reading through the postings and talking to another mx something clicked in my head. What if the mags somehow ended up being timed on the exhaust stroke by mistake?  So I placed the engine on the compression stroke at 25BTDC and looked for the mark on the mag whitness hole and nothing.  After removing the mag we found out that the mag was 180 degrees out of phase so we fixed the issue and reinstalled it.  The engine fired right up and sounds better than ever!!!

Thank you guys for all the help

[carusoam]

Proof that patience and dedication will get you to the goal!

Way to go Isaac!

Fly safely,

-a-

[isaacpr7]

It was a long journey Chris. It took months to get it all done (interiors and ignition) but she's ready to hit the skies. Now I need to find someone to fly with in case my landings got rusty

[Marauder]

It was a long journey Chris. It took months to get it all done (interiors and ignition) but she's ready to hit the skies. Now I need to find someone to fly with in case my landings got rusty

I'm happy for you Isaac! Find a nice day and work yourself back into the saddle. If you feel really rusty, wouldn't hurt to sign up an instructor for an hour.

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