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06-24-2016, 12:14 PM
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Join Date: Jan 2005
Location: Tulsa, OK
Posts: 1,015
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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.
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06-24-2016, 12:39 PM
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Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
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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?
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Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.
Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
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06-24-2016, 01:41 PM
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Join Date: Jan 2005
Location: Tulsa, OK
Posts: 1,015
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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.
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06-24-2016, 03:12 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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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.
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06-24-2016, 05:15 PM
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Join Date: Jun 2014
Location: Ottawa, ON, Canada
Posts: 353
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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?
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J.F.
Sling 4 empennage kit on order!
Future EAA 245 Member (Hopefully)
Current Piper Warrior PA-28-151 Owner/Pilot
Ottawa, Ontario, Canada
http://www.sling4.ninja
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06-24-2016, 09:56 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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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.
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06-24-2016, 10:00 PM
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Join Date: Apr 2008
Location: Lake Havasu City AZ
Posts: 2,393
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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.
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06-25-2016, 06:09 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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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.
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N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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06-25-2016, 06:12 AM
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Join Date: Jan 2005
Location: Tulsa, OK
Posts: 1,015
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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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06-25-2016, 06:19 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by uk_figs
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.
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
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