Mag Rough on Left -- Shuts off on Idle

[base698]

It runs really rough and shuts off on left mag when you do a mag check @ 700 RPM.  Right mag is fine at idle. 

At 1700 RPM, both end up with about 100 RPM drop.   Left is initially rough but ends up at a 100 RPM drop and runs fine.    

Bad mag?  We recently had a top overhaul and some spark spkugs replaced.  Was hoping the spark plugs would fix the problem.

[Hank]

Sounds like your left mag is going. How does the engine start? That's a left magneto only operation, the right is grounded out by the key. 

[Hank]

Time to IRAN the left mag. Wait a while, then do the right one.

Good luck! There are threads on here with recommendations where to send it, and where not to send it . . .

[N201MKTurbo]

Reset the point gap. It will probably fix it. You might want to replace the points. $40 vs $600 for an overhaul.

[Mjknick@gmail.com]

Overhaul don’t mess around with your mags unless your a reputable shop. That’s the heart of your machine. You’ll be glad when it’s up and running smoothly. Money well spent.


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[ArtVandelay]

What does the engine monitor tell you?

[tony]

just check the timing of the mags to see if it moved before you start sending things out of a repair you may not need.  

[Guest]

If the timing to the engine has drifted and the mag is not loose, then the internal timing has changed.  It’s an easy fix as Rich points out above.

Clarence

[Mooneymite]

I was taking delivery of a new-to-me aircraft and the mag check was just as you described.  Turned out the mag was fine once all four screws on the mag case were properly torqued.

 

[Ned Gravel]

Since we are talking about mags here, I have noticed that one of my mags is not producing a 50 degree rise in temperature on my JPI during the mag check.  The temperature increase is closer to 40 - 45 degrees F.   First time in the 12 years I have had a JPI installed.

Should I be concerned about this?

[N201MKTurbo]

11 minutes ago, Ned Gravel said:

Since we are talking about mags here, I have noticed that one of my mags is not producing a 50 degree rise in temperature on my JPI during the mag check.  The temperature increase is closer to 40 - 45 degrees F.   First time in the 12 years I have had a JPI installed.

Should I be concerned about this?

That indicates your timing has shifted. Usually caused by wear in the point cam follower. 

Easy fix is to reset the timing. Better fix is to inspect the points and reset the internal timing and point gap. 

[jackn]

What engine are we talking about?

[carusoam]

Summary...

1) State the hours on the mags in question. What engine they are mounted on... 

(when using a cell phone for MS, it can be hard to tell the avatar info...)

2) Check the timing as it can drift for a number of reasons. Wear is one, unbolted is a more important reason...

3) OH is greatly appreciated after 500 hours on the mags...

 

PP thoughts only, not a mechanic...

Best regards,

-a-

[bradp]

I just had to send a slick mag out at 280 hrs - the issue was that the internal timing (e gap) had shifted.  Symptom was inability to hot start.  Mag was sent to Aaron Williams at Select in Lancaster TX.   Honest, straightforward and he's probably one of the last mom and pop mag experts out there.  My cam follower, spring and carbon brush were worn.  I did an IRAN with him and the cost was very reasonable, all work documented, etc.  Would highly recommend.

[Marauder]

I just had to send a slick mag out at 280 hrs - the issue was that the internal timing (e gap) had shifted.  Symptom was inability to hot start.  Mag was sent to Aaron Williams at Select in Lancaster TX.   Honest, straightforward and he's probably one of the last mom and pop mag experts out there.  My cam follower, spring and carbon brush were worn.  I did an IRAN with him and the cost was very reasonable, all work documented, etc.  Would highly recommend.

I was helping a stranded pilot on my field a few weeks back. Shop was closed over the weekend and we eliminated everything except the mags which were rebuilt less than 60 hours earlier. Same exact symptom, the guy had started it earlier without an issue and flew it here. Went to start after fueling and it would not hot start. Found out from the shop later that the timing had shifted 10°.


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[bradp]

An important distinction is whether the mag internal timing has shifted or the mag to engine timing has shifted.   I’ve found mag hold down bolts loose (finger tight) after service once. That will affect mag to engine timing obviously.   

For egap (internal timing) issues, they may not manifest as an out of time condition on mag to engine timing if there is, for example, minor wear on the cam or breaker points.   What will be apparent is that the mag can not be snapped with the (presumably working) impulse coupling to produce a strong spark below the coming in speed.  How do you measure a strong spark? Take the ignition wire off the spark plug and measure the distance the spark will jump to a good ground source. Less than about 3/8” on a slick will not fire a lycoming hot.   

New star washers should be used for sure each time the mag is removed.  That’s an important safety check.