New Normal Mag Drop?

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guzziguy

Pre-takeoff checklist
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guzziguy
PA28R-180
I have less than 50 hours on two new OH'd mags, new ignition wires, about 100 hrs on a new alternator and all 8 spark plugs were swapped out maybe 3 weeks ago.

However, when I do a mag check, I see a drop of around 150-170 rpm on both sides.
PoH says up to 150 is ok.
But what I don't understand is WHY I am seeing this large of a drop when before on the previous mags, the RPM drops were in the 75 rpm range.

Is this my new normal or is there something else going on?
PS: at the time the spark plugs were swapped out a couple weeks ago, the AP confirmed the timing was correct.

Thoughts?

Thanks
 
If I had new, plugs, new mags, and new leads, I would expect a drop of 80-100 RPM. On hot days (>90F) I would expect at little more due to air density.
 
Presumably no engine analyzer?

EGT recorded on each cylinder would reveal a problem.
 
At run up, try leaning to max RPM before the mag check. If mixture is part of the problem you should see a difference.
 
Kinda hard to pin point without an engine analyzer, but I had a capacitor leak in one of my mags, it OT was showing 150 or so drop in one day, the next day it was 100 rpm drop, until it went to 250 rpm drop and the next day it would just die on the right mag.

But with that fixed, I see the drop now at 60-70 rpm. I do lean during mag check
 
With big drops on both mags you're not getting full power when on Both.

Did someone make a mistake and put short plugs in a head that takes long plugs?
 
Did the mixture change? Temperature can change it....think density altitude.
 
Re-time the mags. I like to pay particular attention to mag timing when doing engine work. The whole system will "work in" as it gets some time on it. I would not be alarmed, but re-timing would be appropriate.
 
Oldmanb777 said:
Re-time the mags. I like to pay particular attention to mag timing when doing engine work. The whole system will "work in" as it gets some time on it. I would not be alarmed, but re-timing would be appropriate.
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I would be alarmed. Something is definitely not right. The OP said that when the plugs were changed a couple of weeks ago, the mechanic confirmed that the timing was right. I believe that. Late or early mag timing doesn't change the mag drop that much at runup RPM; it's weak spark that does it. If the E-gap wasn't set properly in the mags, the spark could easily be weak. If the mechanic used a feeler gauge to set the points instead of the proper indexing too, the E-gap could well be off on both mags.

The OP should look up the TCDS for his airplane and find the specified max static RPM for it for the propeller installed. I bet it won't make max static. At full throttle an RPM below max static could be weak spark or bad mag timing. Or both.

For instance:

upload_2021-6-5_20-51-33.png

Do it over a nice clean area of concrete or pavement, no loose junk. Brakes locked. Hold the nose up with the elevator for a little more ground clearance. Max permissible throttle for a 172 is full throttle. Make sure the tach is accurate first.
 
Dan Thomas said:
I would be alarmed. Something is definitely not right. The OP said that when the plugs were changed a couple of weeks ago, the mechanic confirmed that the timing was right. I believe that. Late or early mag timing doesn't change the mag drop that much at runup RPM; it's weak spark that does it. If the E-gap wasn't set properly in the mags, the spark could easily be weak. If the mechanic used a feeler gauge to set the points instead of the proper indexing too, the E-gap could well be off on both mags.

The OP should look up the TCDS for his airplane and find the specified max static RPM for it for the propeller installed. I bet it won't make max static. At full throttle an RPM below max static could be weak spark or bad mag timing. Or both.

For instance:

View attachment 96979

Do it over a nice clean area of concrete or pavement, no loose junk. Brakes locked. Hold the nose up with the elevator for a little more ground clearance. Max permissible throttle for a 172 is full throttle. Make sure the tach is accurate first.
Click to expand...
Dan, this isn’t my situation, but I can’t help not thanking you for the great advice you provide. I know first hand that you’ve helped me troubleshoot many times! You ‘da man!
 
Update/No Update:
The aircraft is now in for its annual. I'll be sure to mention it to the mech.
And, by the by, I am using a DIFFERENT AP than the one who confirmed the timing and installed the new spark plugs. We'll see how a new set of eyes helps.
 
I vote E-gap also which is when the points open in relation to the magneto vs. when points open in relation to the crankshaft position.
 
Snowmass said:
I vote E-gap also which is when the points open in relation to the magneto vs. when points open in relation to the crankshaft position.
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I am not familiar with what an e-gap is.
Is it something that is adjusted?
Expensive to diagnose/fix?
 
guzziguy said:
I am not familiar with what an e-gap is.
Is it something that is adjusted?
Expensive to diagnose/fix?
Click to expand...
You have to understand how a magneto generates the spark voltage first, and that means understanding alternating current and magnetic flux fields and stuff like that.

But basically, the rotating magnet generates an electron flow in the coil's primary winding. That current, in turn, generates its own field that continues after the magnet has passed the neutral position in the armature: the resultant flux. And that field is what we have to get to collapse very suddenly through the secondary winding, which is wound over the primary, in order to generate a big voltage spike that makes the spark at the sparkplug. The points are a switch that interrupts the flow of that primary current in the winding in order to get that field to collapse, and the timing of the points' opening is critical. It has to happen at a certain number of degrees past neutral to get the biggest, healthiest spark. The magnet by this time is past neutral and is starting to generate the opposite flow in the primary, which means the flux has the biggest drop in its strength and therefore the biggest spark. That number of degrees past full-register is the E-gap.

upload_2021-6-6_17-35-41.jpeg

upload_2021-6-6_17-50-12.jpeg

upload_2021-6-6_17-50-38.jpeg


There are three factors that determine the strength of current generated in an alternator or generator or magneto:

-The strength of the magnet
-The speed of the magnet's field movement (rotation) through the coil
-The number of turns of wire in the coil.

With a magneto, we can't change the strength of the magnet, It's a permanent magnet. The number of turns of wire is also fixed. So we have to fiddle with the speed of the magnet, or with the magnetic field and current it generates in the coil, so we use the points and condenser to do that. They make that field collapse quickly, which has the same effect as a really, really fast-moving magnet. When you turn the mags off, you are just shorting across those points so that as the magnet continues rotating the field gradually changes its polarity, slowly enough that no usable high voltage is generated in the secondary coil. A bad condenser will do the same thing; it's there to absorb the initial bit of current that wants to spark across the points so that the points can open without arcing. Arcing represents continued flow that does the same thing as shorting across the points, weakening the spark. Many folks think that the condenser (a capacitor) is there to prevent the points arcing and burning, but that's only part of it. That magneto is deader than a doornail without the condenser.
 
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Here is what Lycoming says: https://www.lycoming.com/sites/default/files/Magneto Drop-Off.pdf

"Drop-off must not exceed 175 RPM and must not exceed 50 RPM between magnetos.
Smooth operation of the engine but with a drop-off that exceeds the normal specification of
175 RPM is usually a sign of propeller load condition at a rich mixture."
...
"If the RPM drop exceeds 175 RPM, slowly lean the mixture until the RPM peaks. Then
retard the throttle to the RPM specified in step E.(1)(a) or E.(1)(b) for the magneto drop-off
check and repeat the check. If the drop-off does not exceed 175 RPM, the difference between
the drop-off values for both magnetos does not exceed 50 RPM, and the engine is running
smoothly, then the ignition system is operating properly. Return the mixture to full rich."
 
Try doing a mag check with the engine leaned...the drop is much greater than rich. It’s more than just timing.

Although it could be the timing and that’s the first easy thing to check.
 
The OP indicated that the drop was larger than it used to be before the mag overhauls. He likely hasn't done the runup any differently, so a larger drop than previous is concerning.
 
Dan Thomas said:
The OP indicated that the drop was larger than it used to be before the mag overhauls. He likely hasn't done the runup any differently, so a larger drop than previous is concerning.
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So what are you going to do to fix the problem? Of course the word "overhaul" is nebulous but it's hard to imagine that any new replaced parts would be inferior on both mags to the original so the only thing left is adjustment and, on my 50+ year old Slicks, the only adjustments are the internal timing of the mag with the rather silly name of Egap and the timing to the engine.

BTW I have a super accurate method of engine spark timing that involves about 75 cents in equipment.
 
Snowmass said:
So what are you going to do to fix the problem? Of course the word "overhaul" is nebulous but it's hard to imagine that any new replaced parts would be inferior on both mags to the original so the only thing left is adjustment and, on my 50+ year old Slicks, the only adjustments are the internal timing of the mag with the rather silly name of Egap and the timing to the engine.

BTW I have a super accurate method of engine spark timing that involves about 75 cents in equipment.
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Like I said, the e-gap is likely off. E-gap means Efficiency gap, the magic spot that gets the biggest spark.
 
Yup...a piston stop and a free digital inclinometer app for the iPhone will get you 0.1 degree accuracy. I use Angle Pro.
Snowmass said:
So what are you going to do to fix the problem? Of course the word "overhaul" is nebulous but it's hard to imagine that any new replaced parts would be inferior on both mags to the original so the only thing left is adjustment and, on my 50+ year old Slicks, the only adjustments are the internal timing of the mag with the rather silly name of Egap and the timing to the engine.

BTW I have a super accurate method of engine spark timing that involves about 75 cents in equipment.
Click to expand...
 
I tried leaning and re-doing the mag drop test today. Big difference. RPM drop with rich mixture was 175 on each mag. When leaned, mag drop was 75 on each mag.
 
"The new normal" is a bad thing to adopt safety wise. It's what got NASA in trouble on the space shuttle and has been a problem countless times in small aircraft. If something changes, figure out why, don't just rubber stamp it as "the new normal.:"
 
Minor update:
The mechanic performed the runup and could not recreate the mag drop.
WTH?
I asked if he was leaned out when doing the run up: no.
So, I made plans with him for us both to do the run up. So we'll see how that goes.

A thought occurred to me the other day, and I'd like to get the collective feedback on this potential.
Both of these mags are nearly new. Less than 100hrs. Upon installation, I immediately noticed an increase in performance. The engine smoothed out, everything. I was very happy.
Now, bear in mind, RPM drops on the previous mags was around 85 rpm on each side.
Now, the RPM drops were 125-150 on the new mags.

Is it possible that the mags are simply operating THAT MUCH BETTER that when shutting one down results in the higher drop?
 
Factoring in labor to troubleshoot all of this, you could have installed an engine monitor and had much better data, plus good for future issues.
 
OP seems to focus on mags alone as reason for drop.

It’s really about comparing the quality of combustion in the cylinders.

A partially obstructed injector will give you a mag drop on both mags.

Leaning will only increase the drop.
 
Magman said:
OP seems to focus on mags alone as reason for drop.

It’s really about comparing the quality of combustion in the cylinders.

A partially obstructed injector will give you a mag drop on both mags.

Leaning will only increase the drop.
Click to expand...
Actually, I'm reporting the SYMPTOM, not the cause.
I discovered why the A&P couldn't recreate my mag drop: he was running it up only to 1800 RPM. POH says 2300.
A clogged injector screen was cleaned during the annual.
Mag drops are now right at 150 rpm which is within tolerance, but just barely.
 
Magman said:
OP seems to focus on mags alone as reason for drop.
Click to expand...

Yes, and this is what he said in his first post:

However, when I do a mag check, I see a drop of around 150-170 rpm on both sides.
PoH says up to 150 is ok.
But what I don't understand is WHY I am seeing this large of a drop when before on the previous mags, the RPM drops were in the 75 rpm range.

So why would he go fiddling with the injectors? If you have changed something and now the engine isn't running as well, the first thing you should suspect is that thing you changed.

90% of engine performance problems are electrical issues. And 90% of people will try to fix performance problems by fooling with the fuel system.
 
Dan

I did not mean to imply that it was not the mags but only there are other possible
factors as well.

There are things that may have inadvertently been left out of the discussion.

Some are ; type of mag, were they really “ overhauled” and by who and what was drop immediately after install.

I do not disagree with your thoughts on e-gap.
A possible reason for e-gap drift would be failure to lubricate the Breaker Cam.

During my last conversation with Joe Logie he mentioned that all repair procedures are video taped and they can view every mag.

I don’t know know how prevalent this is with other folks but I found that interesting.
 
It should be reminded that the unusually high mag drop is across both mags.
 
Did you follow Lycoming guidance and redo the mag check after leaning?
 
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