Mag Drop Question

For the past 8 years my mag drop test has been pretty consistent, 1800 RPM test left-both-right-both, RPM drop has been around 30 on left and right mags. After my most recent condition inspection in March where I removed and visually inspected the plugs which all seemed OK I now get a 30 RPM drop on the left and 120 RPM drop on the right.

I assumed it was a plug issue after 397 hours and replaced all the plugs today with new tempest plugs (ECI 0-360), the subsequent engine mag drop test resulted in the same results i.e 30 on the left and 120 on the right.

I did the standard compression check, oil/filter change etc on the annual and did not not change anything else.

Any pointers on what I should be looking for or other tests to run? All the normal engine parameters (CHT, EGT, FF etc) are fine in normal operation and oil analysis was also fine. Engine seems to runs smooth on the mag check on both mags.

Puzzled by what might cause the change.

If it runs smooth on just the left or just the right, it is likely a difference in timing.
Did you check or adjust the ign. timing during the inspection?

I did not adjust the timing during the inspection but of course was back there removing/replacing the oil filter and the oil gallery screen so I guess it is possible I moved a mag slightly although they are normally cinched down tight.

I will check the timimg.

Mags

You should also check the magneto breaker points. The points tend to close up over time from wear and this changes the timing. The primary method of setting breaker points is e gap which at a minimum requires some special tools and removal of the mag. After setting e gap, the point gap is verified with feeler gages to be within a certain range. I believe inspection protocol calls for point gap to be verified at inspection. At a minimum during condition inspection the point gap should be verified and the condition of the breaker points inspected. Inspect for contamination and burned points. Badly burned points can also change the timing. Mag to engine timing is standard annual/100 hour inspection protocol.
This is mostly Bendix information but most applies to Slick also. If you hav Bendix there is also an impulse coupling inspection.

I got the same situation on my Cherokee's O-320 ... although it showed after changing 4 cylinders and overhauling the 2 mags. RPM drop differential is 50, where it used to be zero.

The mags were looked at a second time, no reason to believe they're the cause.

We removed the tach and ignition key from the system to check, same problem remains. Verified with an optical tach, problem is indeed there.

Ii am now at the point of suspecting the ignition leads/harness, it's that, or some strange induction issue.

One tip: test the mag drop at different RPMs. see if it occurs everywhere along the RPM range, or not, and if it increases or decreases with some pattern. I understand this can give you a clue as to the underlying cause (Some bright minds here may be able to shed some light).

Since you only did some light work, another thing to check is the ignirion leads themselves. They must be kept squeeky clean from oil/grease, including your finger's skin grease. You could try cleaning them (Champion has a good guide that talks about that ....). In my case I tried and it didn't help.

Doubt your mags moved ... if they moved that easily after playing with the oil filter, you would've noticed before.

Could be the sync (mag to engine), or the point gaps (mag internal) also, and just bad luck it happened at the same time you were doing your inspection.

Start by cleaning the leads. Check the ignition harness (There's a device that lets you check that they're working, though it's mostly just true/false), but even if it's working, doesn't mean it's working right. I'd look there next, maybe when they leads were hanging and being moved around something went amiss (I'm hoping that's what happened to mine!).

How many hours on the engine, was it new and was the harness new?

Engine

It is highly unlikely that this is a plug lead issue. If it was one bad plug lead the engine would run quite rough and drop a lot of r/m. This is much more likely to be a timing problem, probably caused by the breaker points closing or some internal issue. First step should be to check the timing to the engine, if that is ok check the internal timing of the mag. As the breaker point opening moves away from the proper e gap setting the spark will gradually weaken.

Timing

Many years ago I picked up a new Cherokee at Vero Beach. One mag had a horrendous drop. I taxied back to Piper and they went to work on it. A wrench and a soft hammer. No buzz box. Loosen the nuts a bit and hit the mag with the hammer. After several trial and error runs they tightened the nuts and said you're good to go.

120 RPM is a lot for timing unless it's off quite a bit and e-gap or points don't do that overnight. The key here is that it appeared overnight when you did your inspection. It must be something you touched or impacted somehow. Time to re-trace your steps.

Does the engine sound the same on both mags. If not, start looking at ignition wires or the possibility of loose nuts on the mag hold downs allowing you to bump the timing. Also observe EGT's during mag drop for more clues. If it sounds the same, make sure you didn't disturb the P-leads. Your EMS usually takes RPM from both and blends them. When you turn off one mag, it's not blending/averaging anymore.

If the timing problem was a result of point gap wear I would have thought the problem would manifest itself over time rather than a noticeable change after the annual which is the part that is puzzling me. I was also not aware that you are supposed to pull and check the mags at each condition inspection, I have slic mags and thought the next inspection was at 500 hours (I have 397). I did perform the SB a couple of hundred hours ago that replaced the points etc and regapped.

I will check the timing first as that is easy to check.

Thanks for the inputs

uk_figs said:

If the timing problem was a result of point gap wear I would have thought the problem would manifest itself over time rather than a noticeable change after the annual which is the part that is puzzling me. I was also not aware that you are supposed to pull and check the mags at each condition inspection, I have slic mags and thought the next inspection was at 500 hours (I have 397). I did perform the SB a couple of hundred hours ago that replaced the points etc and regapped.

I will check the timing first as that is easy to check.

Thanks for the inputs

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You are correct, e-gap (internal timing, which affects engine timing) changes over time as they wear.

Didn t by chance allow one of the springs in the spark plug lead excape while you had the plugs out did you.? Slick parts usually have springs in the spark plug cap to contact the plug top electrictly.

Do you have CHT/EGT prob and if so, do you see if one cylinder is different than the others. If you have not touched anything else but the sparkplugs, then I would suspect the sparkplug wires since the plugs have been changed with no effect. The plug wire might not be all broken but enough to effect this.

Trying leaning the engine slightly and rechecking. It could be a bit of an mixture issue. Nothing wrong, just if its a bit rich, one mag will commonly have a slightly larger drop that the other. If you suspect ignition do a mag check at full power. If there is a problem, it will show up more at higher power then when checking at just mag check RPM.
Good Luck,
Mahlon

mahlon_r said:

Trying leaning the engine slightly and rechecking. It could be a bit of an mixture issue. Nothing wrong, just if its a bit rich, one mag will commonly have a slightly larger drop that the other. If you suspect ignition do a mag check at full power. If there is a problem, it will show up more at higher power then when checking at just mag check RPM.
Good Luck,
Mahlon

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Mixture issue? How so? Could you elaborate? Which does being a little rich, affect the two mags differently?

Thx

One mag fires the top plug and one fires the bottom. Excessively Rich or lean mixture in the combustion chamber burns differently when lit from the top or bottom thus causing the discrepancy.
Good Luck,
Mahlon

mahlon_r said:

One mag fires the top plug and one fires the bottom. Excessively Rich or lean mixture in the combustion chamber burns differently when lit from the top or bottom thus causing the discrepancy.
Good Luck,
Mahlon

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Alright, good to know ... but this could happen on specific cylinder(s) only? I ask because in my a/c at least, one mag lights the tops on one side, and the bottoms on the other ...

kamikaze said:

... in my a/c at least, one mag lights the tops on one side, and the bottoms on the other ...

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R mag: top plugs R side, bottom plugs L side.
L mag: top plugs L side, bottom plugs R side.

Standard Lycoming practice, AFAIK.

A question for Figs: injected or carburetted engine?

My money is on a duff lead or plug cap (spring or insulator), seeing as they've been disturbed. Had a set of leads go duff after maintenance - two leads at the same time - couple of years ago. Leads were less than 350 hours / 6 years old.

re-read post #2. check the timing.

Mag

BobTurner said:

re-read post #2. check the timing.

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Plus one on this. Why do so many ignore outstanding advice as well as the details of the problem described in the op and the follow up by the op.
The odds are at least 1000 to 1 that a plug lead problem CANNOT produce a SMOOTH mag drop.
The 0 235 L in the Cessna 152 and its near identical counterpart in the Tomahawk are notorious for fouled spark plugs. This always produced a very rough running engine on at least one mag during mag check. A four cylinder engine running on three cylinders cannot possibly be smooth.
All of the IO Lycomings are notorious for fouling plugs due to rich mixture.
this and the 0 235 issue can be largely eliminated by aggressive leaning for all ground operations. Some IO operators go so far as to leave the mixture leaned for descent, approach and landing.

How/why would any mag related timing shift by that much, all of a sudden, when left undisturbed?

A fouled plug doesn't fire hence the roughness ... right?

Though it may not necessarily be the root cause here, i surmise a dirty lead, slightly damaged spring, poor contact, etc. can still cause firing, though with less energy, possibly leading to the symptoms described?

Anyways, at least check the sync, the buzz box test takes 5 minutes ...

kamikaze said:

A fouled plug doesn't fire hence the roughness ... right?

Though it may not necessarily be the root cause here, i surmise a dirty lead, slightly damaged spring, poor contact, etc. can still cause firing, though with less energy, possibly leading to the symptoms described?

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FYI...

Anything that cause additional resistance in path to the plug electrodes, including the plug itself, can/will reduce the energy and quality/size of spark. However, the ignition of the fuel/air mixture is binary. It either lights or it doesn't. The issue is range of conditions under which the spark will ignite the mixture. A weak spark is less tolerant of non-optimum mixture and other conditions which make the mix harder to fire. On the fringes, this often manifests as intermittent firing.

Larry

Check EGT?

I plan on more troubleshooting this weekend, I have a full set of EGT probes so what specifically am I looking for on the EGT test? Should they all rise the same?

I plan on the buzz box test also to see if the timing has shifted.

Not sure how I would test the leads apart from making sure they are connected properly.

Appreciate all the inputs.
Figs

BAD mag, plug, lead

Hi Figs,

I sent you a PM.

Take a look at this:

http://www.aircraftmagnetoservice.net/#!magneto-troubleshooting-guide/c15r3

You may need something like this to check your leads.

http://www.aircraftspruce.com/catalog/topages/cabletester.php

It was a timing issue

Checked the timing today and the right mag was off by about three teeth on the starter ring (3.5 degrees?). Fixed the timing and both mag drops are now the same (about 40 rpm), static full power run up is 2250 with the Catto which is about right and all EGT's rose about the same on the mag tests.

Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.

All HT cables, springs etc seem fine.

Figs

uk_figs said:

Checked the timing today and the right mag was off by about three teeth on the starter ring (3.5 degrees?). Fixed the timing and both mag drops are now the same (about 40 rpm), static full power run up is 2250 with the Catto which is about right and all EGT's rose about the same on the mag tests.

Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.

All HT cables, springs etc seem fine.

Figs

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Glad you figured it out. On a 144 tooth flywheel, each tooth would be about 2.5 degrees

uk_figs said:

Still puzzled as to the root cause as the mage were cinched down and I did not notice a gradual change in the mag drop on the right.
Figs

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Glad you figured it out, Dave. I'd keep an eye on that mag for possible further drift. You may have accelerated cam wear going on. Did you notice higher egt's in cruise? Was it any harder to start? From the document I sent earlier:

A magneto drop that exceeds the allowable limit, but is smooth, with no roughness is in most cases, late engine timing. Cam follower wear makes point opening late.
?Check magneto to engine timing.
The magneto designers have gone to great lengths to have the cam follower, or cam in the case of Slick, wear at the same rate as the point faces. This design feature keeps the magnetos internal timing, ?e-gap?, at the correct angular opening point for long periods of time.
?Inadequate lubrication of cam and cam follower will accelerate wear and result in late engine timing.
?A late spark reduces the sustainable RPM at the mag check. All the fuel is not burned and returned as energy to the piston, but is burned in the exhaust system. Higher than normal EGT will result with late ignition timing.
?On impulse coupled magnetos a broken impulse coupling spring will severely retard engine timing and result in a very large, slowly occurring RPM drop. In some cases the engine will slow down sufficiently to quit. With single impulse coupled installations hard starting will occur.

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