Uneven Mag Drop

[AZOutlaw]

We've been chasing an uneven mag drop for a few months now on an Ovation with IO 550 G.  On the run up, the Left mag will drop and 120 to 145 and Right seems to always be between 40 and 50.  They both used to be in the 40 to 50 range, for years.   We have replaced the spark plugs, rebuilt the magnetos, cleaned the injectors and replaced the ignition leads.  We also did the annual during this and all compressions were good.  Any thoughts on what to look at next?  Ignition switch is best guess.

[Marc_B]

@AZOutlaw if you have a 6 cylinder engine monitor, what are the EGTs doing in the individual cylinders when you do a mag check?  Do you submit data to Savvy?  even if you don't have a subscription you could take a look at your temps using their mag check function to see what's going on with each cylinder during the mag check.  That might help to localize and troubleshoot the issue.

Still could be a magneto issue timing (internal vs external), improperly gapped plugs, faulty plug, corroded ignition harness lead...

https://www.aircraftmagnetoservice.net/magneto-troubleshooting-guide

[N201MKTurbo]

So, it’s either plugs, wires or timing. It could also be the internal timing of the mags. Did this drop start before or after you had the mags serviced?

[Thedude]

Huh I thought them being different was normal, but I guess not...

[Hank]

Check the wires to each plug. I had one go bad on a 2-year-old wiring harness. Mag drop started out like yours, and became greater in several flights. 

[AZOutlaw]

39 minutes ago, N201MKTurbo said:

So, it’s either plugs, wires or timing. It could also be the internal timing of the mags. Did this drop start before or after you had the mags serviced?

It started before the mags were serviced.

[AZOutlaw]

10 minutes ago, Hank said:

Check the wires to each plug. I had one go bad on a 2-year-old wiring harness. Mag drop started out like yours, and became greater in several flights. 

Thanks Hank, but I have brand new wiring as of today.  I think they're good.  

[LANCECASPER]

How many hours on the plugs? Massive or fine wire? Champions or Tempest? Have you checked resistance on each plug?

[AZOutlaw]

2 hours ago, Marc_B said:

@AZOutlaw if you have a 6 cylinder engine monitor, what are the EGTs doing in the individual cylinders when you do a mag check?  Do you submit data to Savvy?  even if you don't have a subscription you could take a look at your temps using their mag check function to see what's going on with each cylinder during the mag check.  That might help to localize and troubleshoot the issue.

Still could be a magneto issue timing (internal vs external), improperly gapped plugs, faulty plug, corroded ignition harness lead...

https://www.aircraftmagnetoservice.net/magneto-troubleshooting-guide

Thank you Marc, I have a JPI EDM 800 but I can look at the EGT's closer tomorrow.  I thought I would put bottom plugs on top too, and tops to bottom, and see if the problem switched to the R mag.....then it's a bad plug.

[takair]

What type of tach do you have….where is it reading from?  Some electronic tachs read from each mag vs a common point like a gear. If you have pickup on each mag, perhaps try to swap them to see if problem follows. 

[anthonydesmet]

Airborne mag check and engine data would also help

[AZOutlaw]

7 minutes ago, takair said:

What type of tach do you have….where is it reading from?  Some electronic tachs read from each mag vs a common point like a gear. If you have pickup on each mag, perhaps try to swap them to see if problem follows. 

I have these and they both tell me the same thing.

[Shadrach]

52 minutes ago, Thedude said:

Huh I thought them being different was normal, but I guess not...

Nope. Ideally, no drop between left and right if the mag timing is perfectly synchronized. Should always see a drop between both and a single mag.

[AZOutlaw]

13 minutes ago, LANCECASPER said:

How many hours on the plugs? Massive or fine wire? Champions or Tempest? Have you checked resistance on each plug?

Probably 8 hours on the plugs.  25 minutes on the wires and the clean injectors.  6 hours on the rebuilt mags.  Plugs are Champion RHB32E, massive and yes, we checked resistance.  I honestly can't remember if he checked them all though.

[Marc_B]

https://www.savvyaviation.com/the-mag-check/

Read up on some of the Savvy information...you can find several articles online.  General gist, your EGTs should be rising on one mag as the fuel air mixture burns slower and so more fuel is left by exhaust stroke.  So if you have one cylinder that's dropping off when running on single mag then it's a plug or wire issue.  If you have a bank of cylinders dropping off or not acting the same it points more towards the mag.

A lean of peak in flight mag check stresses the ignition system more and sometimes help pick up subtle things you might not pick up on the ground.  I had a failing mag that initially passed the mag check on the ground, but failed miserably in the in flight mag check.  Initially looked like 3 different EGTs not doing what all the others were so I thought maybe plugs as they were getting worn.  But when I cleaned and gapped the top right plugs, then it was more obvious that the bottom left plugs were more off...common denominator was the right mag...replaced with a Surefly and haven't looked back!

If you get a chance to upload data with the Savvy mag check take a screen shot if you need help.  I'm sure there's plenty on the forum that would help steer you in the right direction!

[Shadrach]

@AZOutlaw if the drop is not associated with roughness, then you almost certainly have a timing issue. The question is whether it’s internal or external. If this just started recently, I would guess internal. I would remove the mag with the excessive drop and and have it IRAN’d. Your engine monitor data will show an elevated EGT when running on a single mag no matter what.  In your case, bad mag EGT is probably going to show an even more elevated EGT given that what you’ve described reads like the timing is retarded on one mag.

[LANCECASPER]

1 hour ago, AZOutlaw said:

Probably 8 hours on the plugs.  25 minutes on the wires and the clean injectors.  6 hours on the rebuilt mags.  Plugs are Champion RHB32E, massive and yes, we checked resistance.  I honestly can't remember if he checked them all though.

I had an Ovation that would cut out if I taxied with too low of an RPM. The Champions were low hour, but they were junk - 8 or 9 out of the 12 were high resistance. I put Tempest Fine Wire in and it solved that problem evened out mag checks as well.

[PT20J]

17 hours ago, Shadrach said:

@AZOutlaw if the drop is not associated with roughness, then you almost certainly have a timing issue. The question is whether it’s internal or external. If this just started recently, I would guess internal. I would remove the mag with the excessive drop and and have it IRAN’d. Your engine monitor data will show an elevated EGT when running on a single mag no matter what.  In your case, bad mag EGT is probably going to show an even more elevated EGT given that what you’ve described reads like the timing is retarded on one mag.

THIS^^^^^^^^

[AZOutlaw]

18 hours ago, Shadrach said:

@AZOutlaw if the drop is not associated with roughness, then you almost certainly have a timing issue. The question is whether it’s internal or external. If this just started recently, I would guess internal. I would remove the mag with the excessive drop and and have it IRAN’d. Your engine monitor data will show an elevated EGT when running on a single mag no matter what.  In your case, bad mag EGT is probably going to show an even more elevated EGT given that what you’ve described reads like the timing is retarded on one mag.

Everything leads to timing I guess.  It started in JAN and we changed the plugs.  We then sent the MAGS out for rebuild and after a lot of delays got them back and installed with the same result.  After that is was the injector clean out and then yesterday, a new wiring harness.  Waiting for the rain to clear out and then I plan to test it more today while watching EGT's closer.

[AZOutlaw]

Here's some info I got today.  The Left mag side is definitely hotter.  I did the inflight mag check as well.  At first I ran it out to LOP and it was in the high 1400's.  When I did the Left check, everything immediately went off the scale.  The ships single EGT gauge went way past redline so I quickly backed out of that.  I went richer with the mixture and got a lower base EGT and ran the same check.  The right side was smooth and normal.  All EGTS rose some but nothing drastic and the engine was still smooth.  The Left EGTs took off in different directions and the engine got rough.  I had video and at the bottom is a screen shot just before the mag check, the second screen shot is 4 seconds after turning the key.

     

[takair]

I know the mags have been on and off and thus timed.  I think your engine timing is 22+/-1.  Your timing can be in limits, but have a 2degree split and get different mag drops.  If the timing is a little out of tolerance it becomes more significant.  Using the timing cone is not the most precise thing.  I know it’s sometimes hard to ask the mechanic these questions, but perhaps ask him what the mags are timed at…..or suggest retiming them both to 22 or 21?  It might be that the left mag is clocked in such a way that it prefers more advanced and is causing these issues.  Just seems consistent with timing….

[AZOutlaw]

14 hours ago, takair said:

I know the mags have been on and off and thus timed.  I think your engine timing is 22+/-1.  Your timing can be in limits, but have a 2degree split and get different mag drops.  If the timing is a little out of tolerance it becomes more significant.  Using the timing cone is not the most precise thing.  I know it’s sometimes hard to ask the mechanic these questions, but perhaps ask him what the mags are timed at…..or suggest retiming them both to 22 or 21?  It might be that the left mag is clocked in such a way that it prefers more advanced and is causing these issues.  Just seems consistent with timing….

Thank you and that's where we are headed.  Just wondering if there is a better way to time a Mooney than with an antiquated sundial attached to the spinner?  Surely technology has caught up to that too.

[Andy95W]

9 minutes ago, AZOutlaw said:

Thank you and that's where we are headed.  Just wondering if there is a better way to time a Mooney than with an antiquated sundial attached to the spinner?  Surely technology has caught up to that too.

It might be antiquated, but that doesn’t mean it doesn’t work.  Done slowly, and methodically, it is accurate to within 1/2°.  One of the biggest issues is the needle sticking.  That is easily remedied by gently tapping on the needle center to make sure it is unstuck.  Other than that, the design just uses  math and algebra and it works.

[EricJ]

14 minutes ago, AZOutlaw said:

Thank you and that's where we are headed.  Just wondering if there is a better way to time a Mooney than with an antiquated sundial attached to the spinner?  Surely technology has caught up to that too.

On many Continentals there's a plug near the front of the engine that covers a timing mark on the crank.   On Lycomings the flywheel has timing marks that can be sighted for accurate timing. 

[kortopates]

17 hours ago, AZOutlaw said:

Here's some info I got today.  The Left mag side is definitely hotter.  I did the inflight mag check as well.  At first I ran it out to LOP and it was in the high 1400's.  When I did the Left check, everything immediately went off the scale.  The ships single EGT gauge went way past redline so I quickly backed out of that.  I went richer with the mixture and got a lower base EGT and ran the same check.  The right side was smooth and normal.  All EGTS rose some but nothing drastic and the engine was still smooth.  The Left EGTs took off in different directions and the engine got rough.  I had video and at the bottom is a screen shot just before the mag check, the second screen shot is 4 seconds after turning the key.

     

You can provide much more helpful information by downloading the EDM data and uploading it to a free account on SavvyAviation.com.

If still flying it then to get really helpful data, update your engine monitor data sampling rate from the default of every 6 sec to its fastest rate of 1-2 sec and then perform a slow gami sweep, slowly leaning at low power from ROP to LOP and then perform a LOP Mag inflight mag check isolating each mag for a full 30 sec. This will identify which cylinders/plugs/wires and/or possibly point to a mag issue.

If you want some help with the mag test and go all to provide some great diagnostic data review and follow our Savvy Test Flight procedures (it include some info on updating the data sample rate):

Savvy-Flight-Test-Profile-Expanded.pdf