funny business with mag timing = internal mag problem?

[DXB]

I've been chasing a CHT issue predominantly on the leanest cylinder (#4), that was there before my last annual but got worse after annual in both climb and cruise - I've discussed several other CHT factors in another thread but wanted a separate one focused on magneto timing. 

I have a Surefly on the left and a Bendix mag on the right of my O-360-A1D.  Per my A&P at annual, the timing on the right mag was way off, and he advanced it to about 10 degrees to 25 before TDC. Since my overheat issues got worse after that annual, I've scrutinized mag timing carefully.  Before annual, I'd get symmetric mag drop at runup (150 on each side) but after annual the right mag drop changed to only 100, making me think the right side was over-advanced (I'd actually think the Surefly should give you the smaller drop, all other things being equal, due to the longer duration of its spark).  So I had my annual-performing A&P recheck timing on both, and he didn't find anything to be off.  Then I personally rechecked timing on both after watching a Mike Busch webinar on mags and some youtube videos (under direct supervision of a different A&P).  The left Surefly looked perfectly timed (at TDC on #1). The right Bendix mag looked good - somewhere around 24 degrees before TDC on #1.  HOWEVER, there was there was some inconsistency on the right regarding where the points opened when moving the prop back and forth.  Sometimes the box would buzz initially, stop briefly, and then continue while moving the prop - my supervising second A&P said it was very odd and she'd never seen that before. We retimed it based on the very earliest moment the buzz started, but that made zero difference in the asymmetric mag drop or in flight cooling of the engine. I also recalled my first A&P mentioning similar weirdness when he checked the timing of the Bendix mag, with variability in timing light behavior on different cylinders.

My question - does this Bendix mag have an internal issue contributing to misfiring and potential CHT issues?  It is an overhaul exchange mag with <200 hours on it. The engine runs very smooth.  To put this issue to rest, I'm thinking of swapping that mag with a different overhaul exchange mag.   

Or is that just blindly throwing money at the problem without a clear idea what is going on?

[PT20J]

Two things are important when checking timing.

1. Accurately determining TDC. You can use the cone/protractor that attaches to the spinner or a Time Rite. But the best is to use the timing marks on the ring gear.

2. You must back up the prop a half turn and rotate it slowly in the normal direction to note the point where the points open. You can’t rock the prop back and forth (like when checking compression) because the gear backlash will introduce inconsistency.

There is an internal timing called E-gap, but that affects the output voltage more than the timing. It can show up as an excessive mag drop during runup. 

Skip

[RLCarter]

How many hours on the mag  (Bendix)? Worn points will show up as non consistent timing, and as PT20 said, backing up prop introduces all the gear backlash which I would guess is several degrees when all added up

[DXB]

1 hour ago, RLCarter said:

How many hours on the mag  (Bendix)? Worn points will show up as non consistent timing, and as PT20 said, backing up prop introduces all the gear backlash which I would guess is several degrees when all added up

<200 hrs on an overhaul, and yes I’d read about the backlash issue and was carefully supervised by an experienced person while moving the prop so I don’t think that was contributing 

[KSMooniac]

Who did the mag overhaul?

Sent from my LM-V405 using Tapatalk

[DXB]

11 minutes ago, KSMooniac said:

Who did the mag overhaul?

Sent from my LM-V405 using Tapatalk
 

great question, haven't been able to figure out what source the shop used - and that was such a bad service experience overall I don't plan to use them again

[KSMooniac]

There might be a sticker on the mag itself if you can't find an 8130, log entry, or receipt. If it is from Kelly or QAA I would take it off and have a real specialist inspect it.

Sent from my LM-V405 using Tapatalk

[carusoam]

If you have JPI data…

Differences in timing show up as different magnitude peaks during the run-up…

So…. If you have a real difference in timing… real peak differences should be visibly noticeable on the graph…

We have seen this behavior before around here…

Download the JPI data, send to savvy, click the share button, copy the link here…

PP thoughts only, not a mechanic…

Best regards,

-a-

[Guest]

Checking the internal timing is not difficult, any maintainer with some basic tools should be able to do it.

Clarence

[mike20papa]

How old are the plug wires and plugs?  The fact that the Bendix mag is low time and the other is a Surefly is telling me something other than ignition source may be the problem.  

I run nothing but Tempest plugs and you can send your plug wires to Aircraft Magnito Service for testing.

Also, a lean fuel mixture can exacerbate mag drops - and of course  also mess with your CHT's.  Try leaning the mixture when you do a mag check and see if the rpm drops immediately.  You should be able to lean pretty aggressively before an additional rpm drop.   On my o-360 I get barely 50 rpm drop, each mag, full rich.  For me to get a 150 rpm mag drop I would have to lean the mixture pretty aggressively.  (By the way John Deaken on Pelican's Perch use write about the benefits of doing lean run ups/mag checks.)

I've had those buzz boxes act odd.  Remember - they indicate a change in the coil induction rather than a simple breaking of the points.  (and don't ask me what that means!)

Good luck.

Edited August 18, 2021 by mike20papa

[RLCarter]

5 hours ago, M20Doc said:

Checking the internal timing is not difficult, any maintainer with some basic tools should be able to do it.

Clarence

When I was in the motorcycle biz, I always used tech or technician over mechanic, starting to use Maintainer on the aircraft stuff more and more 

[Guest]

1 hour ago, RLCarter said:

When I was in the motorcycle biz, I always used tech or technician over mechanic, starting to use Maintainer on the aircraft stuff more and more 

As Mooneyspace is international, maintainer makes more sense to me than using one countries name.  I’ve used mine, AME, and people assume some form of aviation doctor.

Clarence

[kortopates]

3 hours ago, M20Doc said:

As Mooneyspace is international, maintainer makes more sense to me than using one countries name.  I’ve used mine, AME, and people assume some form of aviation doctor.

Clarence

I thought you were!

[DXB]

On 8/18/2021 at 8:15 AM, mike20papa said:

How old are the plug wires and plugs?  The fact that the Bendix mag is low time and the other is a Surefly is telling me something other than ignition source may be the problem.  

I run nothing but Tempest plugs and you can send your plug wires to Aircraft Magnito Service for testing.

Also, a lean fuel mixture can exacerbate mag drops - and of course  also mess with your CHT's.  Try leaning the mixture when you do a mag check and see if the rpm drops immediately.  You should be able to lean pretty aggressively before an additional rpm drop.   On my o-360 I get barely 50 rpm drop, each mag, full rich.  For me to get a 150 rpm mag drop I would have to lean the mixture pretty aggressively.  (By the way John Deaken on Pelican's Perch use write about the benefits of doing lean run ups/mag checks.)

I've had those buzz boxes act odd.  Remember - they indicate a change in the coil induction rather than a simple breaking of the points.  (and don't ask me what that means!)

Good luck.

Plugs are 250 hr old Tempest UREM37BY, and I clean/gap/resistance test/rotate regularly, replace at around 500hrs. Harness for the Bendix mag is 20 years old and harness for Surefly is new.  However, running on the Bendix mag gives the LESSER rpm drop full rich (100rpm) vs. 150rpm on the Surefly, so it does not seem to be an ignition problem on that side, leading to hypothesis that timing was overadvanced.  150 RPM is the standard drop I'd always gotten on conventional mags as long as I've owned the plane, so the fact that the overhauled Bendix gave less of a drop (even relative to the correctly timed Surefly, which has a longer spark duration) was striking to me.

[David Lloyd]

At some point in ownership, always verify the 0 degree TDC mark on both sides of the flywheel is in the proper place.  Some have been marked wrong in relation to the mounting holes.  Crazy stuff does happen.

With the RV, I was able to change the timing as desired.  At 20 degrees BTDC, there was a very small power loss and CHT were below 375 when at best power, ROP.  25 Degrees BTDC produced the fastest cruise speed by just a couple knots.  CHT would run a little over 400 in climb, and 380-400 in cruise ROP.  About 350 to 360 LOP.  I tried 26-27 degrees BTDC and was shocked by the high CHTs.  In climb, it was difficult to stay under 420 degrees, in cruise as soon as I started leaning the temps would go over 425.   I could do the big mixture pull and run LOP but simply was not worth the constant monitoring.  The engine was an 0-360 with AFP fuel injection and 9.5 compression pistons.  The 9.5 pistons contributed to the high temps as did the advanced timing.  Lesson here: the more advanced the timing, even a little bit, makes a big difference in CHTs.

[Will.iam]

One of the golden rules of troubleshooting is what was the last thing you changed and change it back. If you had 150 rpm drop on your mag and now it only drops 100 rpm i would adjust retard that mag back until you get the 150 drop and then see what your temps are doing on climb out. The 1 or 2 knot loss would be a fair trade to me if it resulted in a 25 degree reduction in cht.

[PT20J]

So, let’s review a few basics. 

The Surefly has two settings: engine timing should be set to TDC and the dip switches set to (in your case) 25 deg BTDC (assuming that this agrees with the data plate on the engine). The dip switch setting can be verified by the flash sequence of the LED.

The Bendix mag has an internal timing (efficiency gap, or E-gap) and engine timing. When the mag is new or overhauled, the E-gap is set. During installation, the engine timing is set. As the mag wears in, the E-gap can change slightly and this can be compensating by “bumping” the engine timing. Properly, the amount and direction (advance or retard) of each bump should be logged. When the cumulative changes in one direction exceed a few degrees (perhaps someone here remembers the spec.) the E-gap should be reset.

A ten degree timing error is HUGE. Either the mag wasn’t originally timed properly to the engine, or there is an issue within the mag.

Also, a 150 rpm drop during a mag check is pretty big. Most airplanes I’ve flown drop around 100.

Skip

[Shadrach]

3 hours ago, PT20J said:

So, let’s review a few basics. 

The Surefly has two settings: engine timing should be set to TDC and the dip switches set to (in your case) 25 deg BTDC (assuming that this agrees with the data plate on the engine). The dip switch setting can be verified by the flash sequence of the LED.

The Bendix mag has an internal timing (efficiency gap, or E-gap) and engine timing. When the mag is new or overhauled, the E-gap is set. During installation, the engine timing is set. As the mag wears in, the E-gap can change slightly and this can be compensating by “bumping” the engine timing. Properly, the amount and direction (advance or retard) of each bump should be logged. When the cumulative changes in one direction exceed a few degrees (perhaps someone here remembers the spec.) the E-gap should be reset.

!A ten degree timing error is HUGE. Either the mag wasn’t originally timed properly to the engine, or there is an issue within the mag.

Also, a 150 rpm drop during a mag check is pretty big. Most airplanes I’ve flown drop around 100.

Skip

Great post. A word about mag drop. Lycoming allows 175RPM drop from “both” to “single” mag operation and a 50RPM between each mag. While legal it is sub optimal in my experience (admittedly mostly with Bendix S20/S200) at least when running two Bendix mags. With care, healthy Bendix mags can be timed to yield less 100RPM from “both” to “single” and have almost no discernible drop between mags. 
 

When does  the surefly system switch to data plate timing? 

Edited August 20, 2021 by Shadrach

[PT20J]

1 hour ago, Shadrach said:

When does  the surefly system switch to data plate timing? 

The 4 cyl Surely comes in two versions, SIM4N which acts like a non-impulse coupled mag, and the SIM4P which acts like an impulse coupled mag with a lag angle to fire near TDC during cranking. Above a few hundred rpm they both will run at data plate timing if set to fixed timing mode. If timing advance is enabled, they begin advancing below about 21” MAP and cruise range rpm. Surefly doesn’t publish the timing map, so these numbers are approximate. There used to be an advance curve on the website, but Jason Hutchison told me that it was more of a marketing description and not technically accurate so they removed it. 

Skip

[DXB]

On 8/19/2021 at 8:49 PM, PT20J said:

So, let’s review a few basics. 

The Surefly has two settings: engine timing should be set to TDC and the dip switches set to (in your case) 25 deg BTDC (assuming that this agrees with the data plate on the engine). The dip switch setting can be verified by the flash sequence of the LED.

The Bendix mag has an internal timing (efficiency gap, or E-gap) and engine timing. When the mag is new or overhauled, the E-gap is set. During installation, the engine timing is set. As the mag wears in, the E-gap can change slightly and this can be compensating by “bumping” the engine timing. Properly, the amount and direction (advance or retard) of each bump should be logged. When the cumulative changes in one direction exceed a few degrees (perhaps someone here remembers the spec.) the E-gap should be reset.

A ten degree timing error is HUGE. Either the mag wasn’t originally timed properly to the engine, or there is an issue within the mag.

Also, a 150 rpm drop during a mag check is pretty big. Most airplanes I’ve flown drop around 100.

Skip

Thanks - My left Surefly is definitely set up and timed correctly, and its associated harness is new, and plugs are in good shape - so I'll take its 150 rpm drop as the gold standard on my particular engine when just running just on it at mag check.  The overhaul exchange R mag was put on 200 hours ago by a trusted shop that permanently lost my trust over unrelated serious issues I had with that annual. My CHTs went up some with the new mag, and so I had a reputable shop look over the timing on it and they adjusted it (not sure what direction). with no discernable effect on CHTs.  Then 12 months later, the next A&P to do my annual found the right mag timing to be more than 10 degrees too retarded (which is massive as you note) - but his correcting changed the rpm drop on that side from 150 to 100 and seemed to leave me with even worse CHT problems (for the first time becoming a serious issue in cruise) and so I was suspicious that it was over-advanced.  But checking the timing hands-on myself while working with another A&P a couple months later seemed to indicate the timing was about right, but we had quite a bit of difficulty figuring out exactly where the points open.  In the end, I think there is something wonky internal to this right mag - a 10 degree shift in timing makes no sense to me based on the E-gap changing.  You've convinced me to pull the right mag and replace with an overhaul exchange from a reputable place (maybe that one in Montana).

[PT20J]

You can’t go wrong with Cliff Orcutt at Aircraft Magneto Service. 

[DXB]

20 hours ago, PT20J said:

You can’t go wrong with Cliff Orcutt at Aircraft Magneto Service. 

So this is basically like a factory new mag that I have right?  I feel a little weird swapping it after <200 hrs….

[PT20J]

If I suspected the mag, I’d send it in for IRAN. I had to do that with a new Slick that came with my Lycoming factory rebuilt because after 200 hrs it was dropping 250 rpm during a run up mag check. Drops about 100 now. Cost about $500.

Skip

[Guest]

2 hours ago, DXB said:

So this is basically like a factory new mag that I have right?  I feel a little weird swapping it after <200 hrs….

Essentially the same as a new magneto.  They are generally very good, but nothing is perfect.  I'd contact Continental tech support before doing anything to it. Fight for warranty.  We got one recently under warranty.

Clarence

[jamesm]

I had a problem that started occasional miss then when away for a few flights a came back. I pulled the left Mag sent it in  it turned out the capacitor/condenser when it was rebuilt at engine overhaul. I had about 200 - 300 hrs on the mag after a rebuild. During run up it would started cutting in and out that is when problem reveal it self in my case but only on some flights. In my case it turned out when it was rebuilt apparently the magneto overhauler  (listed previously who did the overhaul), who was still using remnant stock paper caps left over from war surplus or remnant stock of paper capacitors from the 60's and 70's. So a couple years ago when I had  the right Magneto starting  exhibit similar symptoms I decided it was time to overhaul right magneto which maybe 70 to 90 hrs left before the 500 hr Bendix overhaul requirement. I was  insistence  that overhauler did not use  non paper capacitor preferred them to use tantalum capacitors. My IA and I were installing a Surefly magneto on the left side and newly overhauled Bendix on the right in 2019. Since I was having two relatively unknown components installed, my IA ensured me the that the local Bendix magneto overhauler who rebuilt the right Bendix  mag did run it on the test stand first. This gave me confidence since I just had installed a new Surefly Mag on the left and newly  rebuilt Bendix Magneto on the right would be safe for flight. This combination has been running like a singer sewing machine and trouble free for the last couple of years and happy with functional results.

James '67C