Magneto timing question

SPAJC

SPAJC

Pre-takeoff checklist
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Jeff
I didn't find anything that directly addresses this.

So, the last time I flew my airplane, the mags both checked with a 50rpm drop. It sat for about a week and a half before being flown again by my friend/A&P. He said that it flew great and was a solid airplane,but, asked if I'd had any trouble out of the left mag. I have not, they have always checked the same since I have had the airplane.

He said that during the run-up, the left mag had about a 400rpm drop and that he hadn't looked into it yet, but, that he had tried to burn off any fouling and whatnot and couldn't get it to clear. So, because it wasn't symptomatic prior to, I figured it had a fouled plug and just needed some manual cleaning.

I went to the airport and pulled the plugs. They weren't bad at all, and really no oil fouling to be concerned with.

I run checked it afterwards and it didn't help any to clean the plugs. Had a 500rpm drop.

So I started replacing plugs and such and still no change.

So, I went out today with the thought that the contacts inside the mag might be dirty or burned. I pulled the end cap off and those contacts were good. I didn't try to dig any further than that. But, while putting the screws back in, I grabbed the mag and the entire mag spun very freely.

So my question then is: does the mag itself spinning while mounted have any affect on timing? Seems logical that it would, similar to the distributor on a car....and, since nothing else fixed the drop, would it be reasonable to at least suspect that since the mag moved freely that the problem could have been that it had vibrated itself out of timing?

Obviously I will not be attempting to time the mag. He can do that...along with checking contacts and such if need be.

By the way, the mags only have 83 hours since new.

Any insight greatly appreciated as always!!!


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SPAJC said:
But, while putting the screws back in, I grabbed the mag and the entire mag spun very freely.

So my question then is: does the mag itself spinning while mounted have any affect on timing?
Click to expand...

That is how it is timed to the engine.
 
That's what I thought. But, I have no experience with it, so I defer to you all. Thanks!


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omg yes, the slightest twist to the mag will have the timing way off.
Let's hope it was firing at a time when it didn't matter ie way retarded and not in a way that is harmful to the engine.
Need to inspect and try to find out why it's loose. Let's hope someone just forgot to tighten it, and nothing is broken.
I grab mine everytime I have the upper cowl off to see if they are loose, and try to be fanatical about proper tightening when I have loosened them.
 
SPAJC said:
...But, while putting the screws back in, I grabbed the mag and the entire mag spun very freely...
Click to expand...

In the world of troubleshooting that is what's known as a "smoking gun"
 
It was surprisingly running very smooth even with the rpm drop. Just had the rpm drop. I think it's all good, just needs to be re-timed.

After today, I decided that a check of the mags for security will be included in my pre-flight from now on.

And hopefully you're right and it will just be a matter of re-timing and tightening it back down. I'm going to have my A&P inspect everything pertinent just to be sure.


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I believe I'd sooner have a dead mag than one that was firing at the wrong time.
Did you notice if the hold downs and hardware was still present?
Good luck.
 
Yes it was all there...the bolts weren't loose but probably not torqued adequately to begin with.


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What would be the proper procedure to put that mag back into proper time?
 
If you can make the assumption that it came out of its last maintenance visit properly timed and that the clamp nuts just came loose then just put it at 25 BTDC of #1, hook up your buzz box, set it and secure.
 
Thats what I was thinking would be the way to do it...after reading up on it a bit...25 BTDC on #1, buzz box turned on, right mag should be buzzing with light on, left mag wont be. Turn left mag until it buzzes with light on, lightly secure left mag, fine tune, secure mag, re-install plug(s), wires etc, run check. Correct?


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25° isn't a universal timing figure. Check the engine's data plate.

Dan
 
Right this is just an example, not a tutorial.
 
SPAJC said:
Thats what I was thinking would be the way to do it...after reading up on it a bit...25 BTDC on #1, buzz box turned on, right mag should be buzzing with light on, left mag wont be. Turn left mag until it buzzes with light on, lightly secure left mag, fine tune, secure mag, re-install plug(s), wires etc, run check. Correct?


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Click to expand...
How did you account for the impulse coupling?
 
SPAJC said:
Yes it was all there...the bolts weren't loose but probably not torqued adequately to begin with.
Click to expand...

Or ?

The gasket disintegrated, and all the debris is in your engine? :(
 
to do it as described you have to pull the top plugs and I like to disconnect all the plug leads - maybe that's overkill but you will have the p-leads removed or the mag switch on.
Maybe it would never fire but it just feels all bad to be turning the prop like that.
Be careful of these shortcut procedures.
Honestly if that mag was loose I would pull it off, look inside the engine & other side of the mag, at everything visible including gasket as Tom mention, seating surface of both, impulse coupling, the rubber bushings.
AND check the bolts on the other mag (and anything else that mech touched).

Also you noted a sequence to set the timing... I would add; then spin the prop around again to verify the lights come on at the same time after setting it up as you did.
Good luck.
Stay clear of the plug leads (and the prop) when spinning the prop!
 
Let'sgoflying! said:
to do it as described you have to pull the top plugs and I like to disconnect all the plug leads - maybe that's overkill but you will have the p-leads removed or the mag switch on.
Maybe it would never fire but it just feels all bad to be turning the prop like that.
Be careful of these shortcut procedures.
Honestly if that mag was loose I would pull it off, look inside the engine & other side of the mag, at everything visible including gasket as Tom mention, seating surface of both, impulse coupling, the rubber bushings.
AND check the bolts on the other mag (and anything else that mech touched).

Also you noted a sequence to set the timing... I would add; then spin the prop around again to verify the lights come on at the same time after setting it up as you did.
Good luck.
Stay clear of the plug leads (and the prop) when spinning the prop!
Click to expand...

When you run an engine with a loose mag, will it go retard, or advanced?

(A) it will always lag behind.

So all one must do is bring it back to the advanced position of the other mag.

I'd simply insure the mag switch is off, then rotate the engine forward to the position of the snap of the impulse coupling of the good mag. Then advance the bad mag to its snap of the impulse coupling, lock it down, rotate the engine forward again until the snap of the impulse couplings on #1 cylinder compression stroke, then back it up less than 20 degrees, bring the rotation forward slowly, then tweek the timing to the engine's proper timing with the timing lights. but adjusting the position of the mags on the engine.
No short cuts, the ignition switch stays off until the final step, only 1 plug gets removed to locate #1 compression stroke.
The less you mess with, the less chance you have of messing it up.
 
Oh, allow me to clarify... I will not personally be doing any of this. I'm just trying to understand how its done or what else to look for. I do realize that 25 BTDC was an example and may not be correct for my engine. I hadnt thought of the gasket being bad and all...so good point there Tom. Along with the impulse coupling, of which I am clueless. Going to do a bit more reading on it this evening, but it sounds as if since the mag is already loose, it would make good sense to pull it off and inspect it and the engine prior to re-timing. I kind of had the same thought which was if it was torqued correctly, why did it come loose in the first place...but I havent really put much thought into that aspect yet. This is why I love POA...get lots of different opinions and points of view on things. You guys have answered a good many questions for me on this forum and I appreciate all of the information and insight!


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I do however plan to assist in all of this, so all of your ideas will be addressed. I further agree that the less that gets messed with, the less chance for messing something up. So i guess i need to find a good balance in how far to go vs the fix for the original problem. So, since the mag is already loose, is there anything special about removing it? I've read some about using-call them-"locking pins" the hold the mag internals in place if you remove it. Would that apply here?


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Every time Ive had a mechanic service a mag, it had to be done over again. There may be mechanics that can get it right the first time, but not the ones Ive dealt with. Others have reported similar. Some mechanics advise not touching them. After all, you have two of them. That's my procedure now. So far, its working.
 
coloradobluesky said:
Every time Ive had a mechanic service a mag, it had to be done over again. There may be mechanics that can get it right the first time, but not the ones Ive dealt with. Others have reported similar. Some mechanics advise not touching them. After all, you have two of them. That's my procedure now. So far, its working.
Click to expand...

that's BS.
 
SPAJC said:
I do however plan to assist in all of this, so all of your ideas will be addressed. I further agree that the less that gets messed with, the less chance for messing something up. So i guess i need to find a good balance in how far to go vs the fix for the original problem. So, since the mag is already loose, is there anything special about removing it? I've read some about using-call them-"locking pins" the hold the mag internals in place if you remove it. Would that apply here?
Click to expand...

Change the nuts that hold the mag hardware that secure the mag, to a MS21042-6 nut. The ones that come from the factory that are listed in the IPB are not self locking.
http://www.aircraftspruce.com/catalog/hapages/ms21042.php
 
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SCCutler said:
Tom, I doubt that it's BS; rather, it may be a reflection on his luck in choosing mechanics! :D
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probably too cheap to pay a good one, but that post is typical of the A&P haters of this page.
 
SPAJC said:
He said that during the run-up, the left mag had about a 400rpm drop and that he hadn't looked into it yet, but, that he had tried to burn off any fouling and whatnot and couldn't get it to clear. So, because it wasn't symptomatic prior to, I figured it had a fouled plug and just needed some manual cleaning.
Click to expand...

What does this say about any A&P that will fly any aircraft with that kind of mag drop?

Why in hell didn't he take it back to the shop and fix it ? If he wanted to fly it he should, rather than fly it and give it back with a huge discrepancy.
 
coloradobluesky said:
..Some mechanics advise not touching them. After all, you have two of them. That's my procedure now. So far, its working.
Click to expand...

Well normally if you are "touching" them it's for a reason so I don't see how not touching them is gonna solve that. :dunno:
 
Tom-D said:
probably too cheap to pay a good one, but that post is typical of the A&P haters of this page.
Click to expand...

It is sad when someone apparently has such a poor experience that their perceptions are clouded against the whole profession.

Thank goodness no one feels that way about...

...LAWYERS! :yikes:
 
Ok well.....I defer to the A&P as I am no expert in such things. We went out today and checked everything. All was good, except, the right mag was not torqued either. We found this out as we checked over EVERYTHING....glad we checked. Kinda glad the whole thing happened the way that it did, otherwise it could have been much more catastrophic. Anyways, it wasnt this A&P who installed them or serviced the airplane last. It was the A&P of the previous owner. I trust my A&P and know him to do good work. I also will not stereotype all A&Ps nor anyone else for that matter. Theres good ones and bad ones. Just have to find a good one, which i'm sure is the case more often than not. Just my opinion. Anyways, test flight was excellent. I'm back to 50rpm drop on each. Thanks for all the advice and input!!!


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SPAJC said:
Ok well.....I defer to the A&P as I am no expert in such things. We went out today and checked everything. All was good, except, the right mag was not torqued either. We found this out as we checked over EVERYTHING....glad we checked. Kinda glad the whole thing happened the way that it did, otherwise it could have been much more catastrophic. Anyways, it wasnt this A&P who installed them or serviced the airplane last. It was the A&P of the previous owner. I trust my A&P and know him to do good work. I also will not stereotype all A&Ps nor anyone else for that matter. Theres good ones and bad ones. Just have to find a good one, which i'm sure is the case more often than not. Just my opinion. Anyways, test flight was excellent. I'm back to 50rpm drop on each. Thanks for all the advice and input!!!


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Click to expand...

Glad you got it fixed.
 
Anyways, it wasnt this A&P who installed them or serviced the airplane last. It was the A&P of the previous owner.
Click to expand...

Can we be sure it was not the previous owner (vs his A+P) who left them loose?
Owners have been known to work under the hood as well.
 
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Thanks Tom! And no, i can't be sure of that. Last log entry is from the mags being installed during a 100hr inspection done by a A&P/IA. It could very well have been the previous owner. No way to be sure. But it's fixed and correct now. Thanks again for all the input!
 
As Tom pointed out magnetos are secured with non self locking nuts. This is not a problem providing you use the proper lock washers and understand that lock washers are not meant to be re-used so put new ones on.
 
A lot of guys are sort of intimidated by magnetos... When I started working on GA planes I was a little, and then I sat down and did the internal inspection on a couple. Once you do this, you realize that they aren't much more complicated than a lawn mower combined with a 1950's car distributor... The only thing that really sucks is the parts cost 10x than the ones to work on said lawn mower or old car:yikes:

Glad you got it fixed, and nothing expensive was the issue!

-Dana
 
Dan Thomas said:
25° isn't a universal timing figure. Check the engine's data plate.

Dan
Click to expand...

I had an interesting conversation with Mike Busch at OSH on this very subject. Lycoming dialed back some 0-360s to 20 degrees BTDC because of high CHTs. I am having high CHTs on an XIO-360 so he advised me to dial back the timing from 25 to 22.5 degrees BTDC and see if that helps. Very knowledgeable man.

Never understood why mag nuts are not lockable.
 
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Geico266 said:
Never understood why mag nuts are not lockable.
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I guess the engine makers have found that lockwashers are enough, as long as they're properly torqued in the first place. Connecting rod nuts in many engines aren't locknuts and have no lockwashers or Loctite, either, the nut being restrained by the pressure maintained on it by the stretch in the stud. Some older engines used castellated nuts and cotter pins. My Gipsy Major had brass lockwire on the rod nuts. It also had indexable vernier magneto drive couplings for timing, a pain to set but could never slip.
 
Lock nuts are difficult to properly torque presenting an increased risk that they will either be inadequately tightened or over tightened and damage the threads in the accessory case that the stud is screwed into. Frankly for magnetos, which need periodic adjustment or removal I think lock nuts would be a PITA. They aren't necessary despite the couple of stories here about loose mags. Those instances aren't because of the lack of lock nuts, they're due to a maintenance error by someone.
 
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