What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Excessive mag drop

Flying Canuck

Well Known Member
Patron
I've got 2 regular Bendix magnetos. I've been chasing down an issue with the help of my engine builder where the right mag is seeing drops of 300+ rpm pretty much every time I do a mag check. The plugs are all good, the coil is new. The mag runs perfectly on the bench.

My engine guy is checking the capacitor, in the meanwhile I was wondering if anyone might have any ideas. Could a faulty p-lead cause this? How about a connection to my engine monitor (p-lead pickoff with an inline resistor)?

For information sake, my left mag sees a drop of about 100. I check my mags at 1800 rpm. I have no issues while flying. My engine is an IO-320.

Thanks for any insights.
 
What do you mean "Runs normally on the bench"? Do you have a machine that spins it up to rpm? How about sending it out to an accessory shop so they can perform a IRAN on it? When's the last time it was IRAN or overhauled by an accessory shop? There's a lot more to a mag than timing. .there's the points and capacitor, internal timing, dwell...
 
BobTurner said:
Have you checked the timing?

Yes, it's been checked and it's good.


What do you mean "Runs normally on the bench"? Do you have a machine that spins it up to rpm? How about sending it out to an accessory shop so they can perform a IRAN on it? When's the last time it was IRAN or overhauled by an accessory shop? There's a lot more to a mag than timing. .there's the points and capacitor, internal timing, dwell...

To be honest, I don't know what I mean by that. It is my engine builder who is testing it, off of the plane. He rebuilds these things with some frequency and has 50 years of experience with them. While I can't quote his language in this forum, he tells me it sparks very well in his testing setup. He doesn't know what the issue is yet, but I'm fairly confident that if it is an internal mag issue he will find it. What I'm really wondering is if any external items would be able to cause this issue.
 
Does the EGT indicate a plug not firing?

If not, is the engine running smooth despite the large RPM drop?
 
Does the EGT indicate a plug not firing?

If not, is the engine running smooth despite the large RPM drop?

In an unfortunate coincidence, I had a failed EGT probe for the past while. I replaced it 10 days ago, and I haven't flown since (have confirmed that it works). As such I don't have reliable EGT information to look at. The temps on the 3 working probe were all similar, so nothing to suspect there. If there was a problem with #2 that was masked by the bad probe, I don't know. I did have the plugs checked and cleaned, replaced 4 spark plugs (also in the past 10 days). The mag drop continued after that work, although my left mag went from ~175-200 to ~100 with the new plugs. The Champion plugs that were replaced failed testing so we put in new Tempest plugs (the other 4 are also Tempest).

Engine is running smoothly in the air. Warm ground running is rough, but I can ease that with aggressive leaning.
 
I had this same symptom and problem once on a PA-28 I flew in a club.

After rebuilding the mag twice, they finally diagnosed it correctly.

They told me it was bad wires.
 
Lycoming SB 1132B says you should lean mixture for max rpm, reduce throttle back to mag check rpm, check again. If within specs (max 175 rpm drop, two mags within 50 rpm) then all is well.
 
Lycoming SB 1132B says you should lean mixture for max rpm, reduce throttle back to mag check rpm, check again. If within specs (max 175 rpm drop, two mags within 50 rpm) then all is well.

Not possible to lean for rpm with my C/S propeller (at least I don't know how). Mixture doesn't seem to affect this particular issue. If the engine is warm, I need to lean aggressively for smoothness. I don't tend to mag check when full rich. My left mag is generally 175 or less, right mag is at least 300.

I should add, I have separate mag toggle switches, when I am checking the right mag, that is to say the right switch is on and I'm cycling the left switch.
 
Not possible to lean for rpm with my C/S propeller (at least I don't know how)

With prop control full forward the prop should be against the fine pitch stop, until and if you get to RPM red line. Increasing power from 1800 rpm (whether the throttle, or setting best power mixture) should increase rpm.
Set 1800 rpm.
Lean mixture to maximize rpm.
Reduce throttle back to 1800 rpm.
Re-do mag check.
 
That makes sense Bob. I was thinking more about leaning at full power, which wouldn't work because the governor is engaged. I think that happens around 1900.

I'll give that a try when I get the mag back.
 
That makes sense Bob. I was thinking more about leaning at full power, which wouldn't work because the governor is engaged. I think that happens around 1900.

.

If the prop control is full forward the governor doesn’t ‘engage’ (open valve allowing oil pressure into prop) until 2700 rpm.
 
Okay I just need to stop saying things that show how much I need to learn. I have the impression that when I pull power to 15" or lower for short final, the RPMs come back without touching the prop control and settle in around 1900. Any time after that I can push the prop in without effect. That's behavior slowing down, I've never really looked at the power/prop interaction when advancing the throttle. It usually only a few seconds during takeoff roll, my attention is elsewhere. I do recognize that RPMs advance with throttle, I hadn't thought of the governor's (lack of) role while advancing the throttle. Makes sense the the governor doesn't do any governing until you pull on the prop control. There's a power setting where the governor can't hold the RPM anymore regardless of the position of the prop control.

Thanks for the education.
 
Last edited:
Sparkplugs

I would swap the plugs, bottom-to-top, top-to-bottom, on each cylinder.
Then do a mag check and see if the excessive drop has moved to the other mag. If so, then it is a matter of replacing bad sparkplug(s).
 
I would swap the plugs, bottom-to-top, top-to-bottom, on each cylinder.
Then do a mag check and see if the excessive drop has moved to the other mag. If so, then it is a matter of replacing bad sparkplug(s).

Bottom plugs are brand new, top plugs just tested. The top plugs aren't likely to be in the same position as they were before my annual. The issue got better after the plug checks and replacements - it was pushing 400 before. The replaced plugs all failed a resistance check, so an improvement is to be expected. The problem still remains though.
 
I still think you should do what OKAV8R suggested. Swap all plugs from top to bottom, check again. If the issue stays with the same mag, then it's not the plugs. Even new plugs can be bad.

Next step it to check your spark plug harness. Take the wires off of the spark plugs, and take the cap off the mag. Then do a resistance check on each center wire on all 4 wires. Should be 0-3 ohms or so. Also test the wires with a spark plug wire tester, which uses high voltage to look for an issue. These tests are all a lot cheaper than overrhauling a mag.

I always look for the cheap solution first! :eek:
 
I still think you should do what OKAV8R suggested. Swap all plugs from top to bottom, check again. If the issue stays with the same mag, then it's not the plugs. Even new plugs can be bad.

Next step it to check your spark plug harness. Take the wires off of the spark plugs, and take the cap off the mag. Then do a resistance check on each center wire on all 4 wires. Should be 0-3 ohms or so. Also test the wires with a spark plug wire tester, which uses high voltage to look for an issue. These tests are all a lot cheaper than overrhauling a mag.

I always look for the cheap solution first! :eek:
All this, plus be sure to move the spark plug wires around when testing - sometimes they work perfectly in one position, but not in another.
 
I had a similar issue

A couple years ago, I had a similar issue but mine was intermittent. After a few lean outs I could get the drop to go away, but then it would return, and a couple times I had a roughness in the engine when power was pulled back for pattern entry. I pulled plugs and gave quick exam with no findings, but problem remained. Plugs back out and detail clean/inspect..found a cracked insulator that could be moved with my cleaning pick. New plug and problem solved. I had never dropped or abused the plugs and have no idea what cracked the insulator, but it was the smoking gun fix.
 
Excessive RPM drop is often the result of timing mismatch between mags. If the x mag is too far advanced, relative to the Y mag or the y mag is too retarded relative to the x mag, then when the x mag is grounded via the p lead, the RPM drop will be excessive. There is a direct correlation between advance and RPM at run up RPMs, at least with the relatively moderate advance of fixed timing mags.

I would re-verify the timing on both mags. It is usually the timing differential that causes the excessive drop and the mag that produces the large drop when it is taken off line has more advance than the other one.

A bad condenser is not likely to cause an excessive RPM drop. More likely to cause either misfires or pitted points. It shouldn't impact timing. Their easy to test with a good meter that has a capacitance setting.

If you had one plug not firing, one of the mag tests should create a serious roughness with a complete fall off in EGT on one cyl.

Larry
 
Last edited:
About your Bendix mags, were they installed new or refurbished/overhauled? Couple of years ago I was chasing ghosts responsible for a recurring mag drop problem. Turned out to be an overhauled (not refurbished) mag that had not been updated to include the latest Bendix SB for rotor arm material. I will never install another “overhauled” accessory in my plane.

I can't really say. I got them from my engine builder, they are certainly not new, but were "zero time" as in 500 hours left on them. I am not privy to mysterious ways of the mind of my engine builder. The good thing about it is that he's not charging me for anything and appears committed to resolving it.

I'll have a look at my engine log next time I get to the airport and see how he labeled the mags.
 
+1 Check the harness

I second the idea to check the harness, or even borrow a known good one for testing. I had a mag drop problem, and we went through all the expensive things first :rolleyes:, insisting it couldn't be the brand new harness from Penn Yan with 120hrs on it. Finally replaced the harness (no visible issues, resistance check was fine, etc) and it fixed the problem. Luckily covered under warranty, but frustrating.
 
Excessive RPM drop is often the result of timing mismatch between mags. If the x mag is too far advanced, relative to the Y mag or the y mag is too retarded relative to the x mag, then when the x mag is grounded via the p lead, the RPM drop will be excessive. There is a direct correlation between advance and RPM at run up RPMs, at least with the relatively moderate advance of fixed timing mags.

I would re-verify the timing on both mags. It is usually the timing differential that causes the excessive drop and the mag that produces the large drop when it is taken off line has more advance than the other one

This is exactly what I saw a while back...at first, there was a small difference (where there had been almost none before), then over about 6 months, it got to about a 75 RPM difference. Time to fix it...turned out that one mag (I forget which) had advanced just a degree or so (internal timing drift); retimed, problem gone.

Besides, checking timing is easy and free, so might as well start there :).
 
Claude -
maybe this is the perfect time to think about changing up to an electronic ignition & throw that problem mag away. There are great choices out there and you would get the benefits of a solid ignition, and added efficiencies that EI offers.

You could even take your 'no fly time break' to drive down (2 hrs south) & visit us at Springbank and visit with Ross at SDS ignitions, you could view his CPI & CPI2 ignition offerings, as well visit me as I'm only 5 buildings north of him.

Cheers
 
Last edited:
Claude-
actually, I may have a second option for you. I have a low time engine with Slick mags, I could sell you the one coming off that, as I am looking for an excuse to put a CPI ignition on that plane...
 
Claude -
maybe this is the perfect time to think about changing up to an electronic ignition & throw that problem mag away. There are great choices out there and you would get the benefits of a solid ignition, and added efficiencies that EI offers.

You could even take your 'no fly time break' to drive down (2 hrs south) & visit us at Springbank and visit with Ross at SDS ignitions, you could view his CPI & CPI2 ignition offerings, as well visit me as I'm only 5 buildings north of him.

Cheers

Hey Ralph!

This sounds like a good idea. No sure I want to sink much more $$$ right now, but it's worth looking at.

I don't know much about electronic ignition, do you need redundant battery power with them? Do they use the same plugs? Can you mix them with an IC mag? How's the installation?

The Slick mag you mentioned is also intriguing, can that mix with a Bendix IC mag?

I tried to find your number a couple weeks ago, I found myself in Springbank with a storm inbound threatening heavy rain and hail. I was scrambling for a spot in a hangar while it blew over. Managed to find a spot and the storm didn't materialize. Regardless of what I'm working on, I'd love to come down and visit.
 
I see no problem running a Slick & a Bendix on the other side (Any adverse reason why not out there anybody?). Slick & Bendix mount the same way, timed the same way. You will have to also replace the plug wires too, which I have.

As far as replacing the right mag with a electronic ignition and leaving the Bendix on the other side, I have been doing this for many years & have installed similar on many other planes. No Hiccups yet. I actually think switching to dual electronic ignition is overkill & adds so much more complexity (my opinion anyway...). Wiring in a single CPI does not have to be complicated.
 
Yes, it's been checked and it's good.

To be honest, I don't know what I mean by that. It is my engine builder who is testing it, off of the plane. He rebuilds these things with some frequency and has 50 years of experience with them. While I can't quote his language in this forum, he tells me it sparks very well in his testing setup. He doesn't know what the issue is yet, but I'm fairly confident that if it is an internal mag issue he will find it. What I'm really wondering is if any external items would be able to cause this issue.
50 years experience? How old is this fellow? An accessory shop that rebuild mags all day has bench equipment to spin them and test output off the engine. How much RPM drop are you getting? 300. That is NFG.

If you suspect a p-lead remove it temporarily.

Did you test compression?

How do you know plugs are "good". Even new plugs can be bad, Have you pulled all your plugs, cleaned them, gapped them and tested them under pressure. Your guy should have a spark plug bomb tester. You put it in the machine and apply shop air pressure. It provides a high voltage spark to the plug and there's a window to view. There should be a strong spark.

Someone mentioned timing. That should have been checked long ago.

Do you have an engine monitoring system that shows EGT mchd on all cylinders? If you can capture that data it might be of interest in diagnosing the issue.

Harness. That also can be tested. Or somebody has a spare harness you can swap them out and see if that helps.

Last would be to take the Mags out and send them to a repair shop to have them tested. I believe there's a 600 hour reoccurring a d on these mags. If that hasn't been done maybe you should consider having them overhauled or at least having the ad complied with.
 
Last edited:
Plugs !

I'll second "Even new plugs can be bad" . I have found in the last couple of years one in ten plugs are infinite resistance (open). Maybe i'm buying from the same batch stock?
Each one is checked new or very good used the same way before install. John
 
50 years experience? How old is this fellow?

Old? Yes I guess he qualifies. He is an old piston engine guy that got his start working on DC-3s, PBYs, A-26s, Convairs, Beavers, Otters, Norsmans etc & many many little opposed 4 bangers. He probably hasn’t had the chance to follow up on the problem yet.
 
Last edited:
Old? Yes I guess he qualifies. He is an old piston engine guy that got his start working on DC-3s, PBYs, A-26s, Convairs, Beavers, Otters, Norsmans etc & many many little opposed 4 bangers. He probably hasn’t had the chance to follow up on the problem yet.

Yeah, Dave is in his mid 70s and has only one gear. He's worked on and flown countless types and is a walking encyclopedia. His time is a little hard to pin down, but he is working on it. Last I heard he had the mag in his shop and was giving it a good look.
 
50 years experience? How old is this fellow? An accessory shop that rebuild mags all day has bench equipment to spin them and test output off the engine. How much RPM drop are you getting? 300. That is NFG.
Yes, I don't think the 50 years is too far off, might even be more.
I'm getting 250+ drop, it has pushed 400. Appears to have improved somewhat with the new plugs - although I've only got one sample. It was still 250 or more.

If you suspect a p-lead remove it temporarily.

This might well be the issue.

Did you test compression?

Not officially, but two old timer AMEs have no concerns after feeling the resistance when turning the prop.

How do you know plugs are "good". Even new plugs can be bad, Have you pulled all your plugs, cleaned them, gapped them and tested them under pressure. Your guy should have a spark plug bomb tester. You put it in the machine and apply shop air pressure. It provides a high voltage spark to the plug and there's a window to view. There should be a strong spark.

The reason the bad plugs were replaced was because they failed the testing. The plugs were cleaned, gapped and tested in his tester.

Someone mentioned timing. That should have been checked long ago.

Timing was checked 3 months ago. Left mag was adjusted, right was already good. Both are at 25 degrees +/- 0.2.

Do you have an engine monitoring system that shows EGT mchd on all cylinders? If you can capture that data it might be of interest in diagnosing the issue.

I do have partial EGT data, had a bad probe so only have 3 cylinders for the last handful of flights. Probe replaced and working now, but again only one sample and I don't have that data downloaded.

Harness. That also can be tested. Or somebody has a spare harness you can swap them out and see if that helps.

Harness has been tested with a high voltage cable tester.

Last would be to take the Mags out and send them to a repair shop to have them tested. I believe there's a 600 hour reoccurring a d on these mags. If that hasn't been done maybe you should consider having them overhauled or at least having the ad complied with.

The mags have been out for testing, the problem out is currently out. They have 180 hours on them, I know they weren't new, but the were "zero time". Not sure if that's an overhaul or rebuilt or refurbished, and I don't know what the distinctions are with those anyways.
 
Internal Timing

OK, from my one attempt to build up my own (slick) mag... there's such a thing as "internal timing". Timing the engine will not show if the internal timing is right or not. Meaning the mag can show as being properly timed, but be incorrect on 'internal' timing. As I recall internal timing affects how strong of a spark the mag will generate. A mag could also have failing internal parts that lead to weak spark.

https://www.kitplanes.com/magneto-timing/

Please read the B-17 "909" report (NTSB crash report) for a chain that started with mag issues.
 
Finally managed to get my post maintenance flight done. Dave changed the capacitor in the right mag. He couldn't find anything wrong with it at all (he's only got 45 years of experience with mags). I did a quick resistance check on the p leads, detached from the mags - both are at about 0.3 ohms with the mag switched off (center conductor to outer sheath) and open when the mags are switched on.

After the usual stubborn start after maintenance, the engine got up to a good rhythm at idle, below 900 rpm. I leaned for best rpm at 1800 and checked the mags. Left was 90-100, right was 160-200. Definite improvement and just within spec. Flight was uneventful, engine ran well. I did a couple of mag checks at 2400 rpm/24". Both times the engine had noticeably more sound change with just the right mag, but didn't sound like it was struggling.

In all a good test and my confidence in the ignition system is greatly improved - which was the main objective.

Thanks for all of the input. I'll be watching the right mag closely and if it degrades again I'll likely look at replacing something or other.
 
I would again check the "e-gap" timing (internal). Most Bendix mags have a point gap set to open when shaft is 15 degrees past e-gap. You may find a Bendix point protractor paper scale online, or it is easy to cut a plastic one to fit. If there is excessive mag drop with no coincidental roughness, it is usually point cam rubbing block wear (i.e. too little gap, late internal timing) causing weaker spark.
Good place to start.

DAR Gary
 
I would again check the "e-gap" timing (internal). Most Bendix mags have a point gap set to open when shaft is 15 degrees past e-gap. You may find a Bendix point protractor paper scale online, or it is easy to cut a plastic one to fit. If there is excessive mag drop with no coincidental roughness, it is usually point cam rubbing block wear (i.e. too little gap, late internal timing) causing weaker spark.
Good place to start.

DAR Gary

spark is mostly binary. It is either strong enough to light the charge or it isn't. Most common symptom of weak spark is intermittent misfires, typically when lean or high altitude. Also, I would really hope that a 45 year mechanic wouldn't miss something so basic to mag maintenance.

I still believe it is a timing mismatch and a traditional auto timing gun would confirm or disprove that. If there is a lot of slop somewhere or excessive backlash, the buzz box approach to timing can create variances once the engine is spinning at operational RPMs. That fact that pulling both mags and reinstalling them, without significant mag work, reduced the delta, kind of points to that. I don't believe a bad condensor can affect timing or mag drop delta.

Larry
 
Last edited:
I would again check the "e-gap" timing (internal). Most Bendix mags have a point gap set to open when shaft is 15 degrees past e-gap. You may find a Bendix point protractor paper scale online, or it is easy to cut a plastic one to fit. If there is excessive mag drop with no coincidental roughness, it is usually point cam rubbing block wear (i.e. too little gap, late internal timing) causing weaker spark.
Good place to start.

DAR Gary

This is not a weak spark issue at all. The spark is very strong. The coil was replaced the first time the mag was looked at a couple of months ago because of poor spark. Internal timing was checked this past week and is set at 25 degrees.
 
OK, so "external" timing may be set correctly at 25 degrees, however the internal timing (e-gap) is the point opening at the highest break down of the coil for the hottest spark. As the rubbing surface of the points wear on the cam, or in the case of Slick mags, the cam wears (plastic), the point gap narrows, the internal timing becomes retarded, and weak spark follows.
I have found a lot of Slick Mags with very worn cams, and points that barely open, soon to fail. Worth a look, use proper cam lubricant per the manufacture.
Regards,

DAR Gary
 
dwell angle

OK, so "external" timing may be set correctly at 25 degrees, however the internal timing (e-gap) is the point opening at the highest break down of the coil for the hottest spark. As the rubbing surface of the points wear on the cam, or in the case of Slick mags, the cam wears (plastic), the point gap narrows, the internal timing becomes retarded, and weak spark follows.
I have found a lot of Slick Mags with very worn cams, and points that barely open, soon to fail. Worth a look, use proper cam lubricant per the manufacture.
Regards,

DAR Gary
Aren't you referring to dwell angle (internal timing or e-gap)? On a Slick Mag with new points, what should the dwell angle be? After 500 hours, what would you expect the dwell angle to be considering the wear on the rubbing block?
 
Bendix points should open 15 degrees after "E" gap. If the internal timing is off (usually due to a worn point cam, usually caused by lack of lubrication, then even if the engine timing is spot on, the mags will not put out the required energy to quickly ignite the fuel.

DAR Gary
 
Aren't you referring to dwell angle (internal timing or e-gap)? On a Slick Mag with new points, what should the dwell angle be? After 500 hours, what would you expect the dwell angle to be considering the wear on the rubbing block?

Dwell angle is different, but similar. Dwell is about the time that the points are closed, insuring that the coil has enough time to charge from battery voltage and not too much time that it over saturates. A mag is unique, as it uses a spinning magnet to generate power and there is only meaningfull power output at two poles on the armature. Egap sets the points so that they open just after that pole passes a certain point so that the coil's primary winding reaches the greatest saturation level (it can't oversaturate like an auto setup) and therefore the greatest output from the secondary winding at points opening. As points and cams wear, this must be adjusted and that adjustment is completely unrelated to ignition timing.

In an auto system, the coil charges (via a constant battery source) at a lower current for a longer time period, therefore we are concerned with how long the points remain closed and that is why a minimum and maximum dwell angle is specified. In a mag, ALL of the power comes in just 10-15 degrees of armature rotation. If that window is not in the specified alignment with points opening, the coil charge can get very low. It is not about dwell time or angle, but instead about having the points open event at the right position relative to the armature's magnetic pole.

Larry
 
Last edited:
Back
Top