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Thread Title: Magneto problems
Created On Sunday 7, August, 2011 6:52 PM
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Gaucho
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Sunday, August 07, 2011 6:52 PM

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Yesterday, prior to take off I did the usual magneto check. The Left magneto was OK, but the Right one dropped 100rpm and the engine missfired. We switched all the sparkplugs, the front ones (Left magneto) to the back, and the back ones (Right magneto) to the front. But the Right magneto continued with the same problems. Any thoughts or suggestions before taking off the harness to check it or before taking off the Right magneto?
Thanks.

-------------------------
Patricio Seidel
Buenos Aires

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tntmck
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Sunday, August 07, 2011 8:46 PM

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What kind of engine and mags

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Rene
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Monday, August 08, 2011 6:45 AM

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Try removing the back cover off the mag and dry with a hair dryer. or take an enclosed shielded light blub and leave it in overnight. i suspect if everything had been fine before, you are probably experiencing moisture in the mag and it is shorting out. Lot of humidity will do that. you can leave a sixty watt blub in the engine compartment and it will keep everything nice and dry.


-------------------------
Rene St. Julien SRA 3882
15351 Navion Drive
Port St. Lucie, Florida 34987
Treasure Coast Airpark FL37
772 708 9210

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Bones
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Monday, August 08, 2011 7:54 PM

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By the way - isn't the front plugs = right hand mag and rear plugs = left hand mag? This is how it is on my W670?
There are lots of old posts on magneto problems and not to state the obviousb - moisture is a big player


-------------------------
John Tabone SRA# 3535

Dee Why, Australia

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ewarning
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Tuesday, August 09, 2011 8:42 AM

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I spent the last few months trouble shooting a mag issue. How old are the mags and how long has it been since they have been overhauled? Mine were over 20 years since overhaul and had around 500 hours on them when one failed. I ended up replacing both to get away from the auto advance tank mags. My W670 seems to run much smoother with the VMN7DF mags vs the VMN7DFAs.

Good luck,

Eric

-------------------------
Eric Warning SRA 4864
Frankfort, Illinois
1942 PT-13D N747RA
eric.s.warning@boeing.com

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Gaucho
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Tuesday, August 09, 2011 5:10 PM

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Thanks for the tips. Keep them coming!!!
I have the standard Continental engine, with Scintilla magnetos.

-------------------------
Patricio Seidel
Buenos Aires

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Gaucho
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Wednesday, August 10, 2011 8:23 AM

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I think John is correct: the Left magneto is for the rear plugs and the Right magneto for the front ones.
Tommorrow I'm going to the hangar to test the harness for continuity, the plugs, etc. Will take a hair blower.
Some non-Stearman pilots suggested doing an in-flight magneto check, at normal cruising speed. Any of you tried it?

-------------------------
Patricio Seidel
Buenos Aires

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Rene
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Wednesday, August 10, 2011 8:28 AM

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a hair dryer will dry the mags out, but if you leave a shielded 60watt lamp in the accessories compartment with the door left down it will have removed all of the humidity overnight. Rene

-------------------------
Rene St. Julien SRA 3882
15351 Navion Drive
Port St. Lucie, Florida 34987
Treasure Coast Airpark FL37
772 708 9210

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Rene
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Wednesday, August 10, 2011 8:31 AM

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checking the mags in flight at cruise , no big deal. Once you are in cruise it should be fine because of the warmth of the engine. But you should not takeoff if the problem is
still evident on the ground.

-------------------------
Rene St. Julien SRA 3882
15351 Navion Drive
Port St. Lucie, Florida 34987
Treasure Coast Airpark FL37
772 708 9210

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Tex
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Wednesday, August 10, 2011 8:51 AM

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"an in-flight magneto check"? Are we serious here? I can't think of any reason to "test" the magnetos in flight. Seems to me like playing Russian Roulette. That's just an unnecessary risk.

IMHO if a mechanic can't find the problem on the ground he is a hack. Find another mechanic.

-------------------------
Phil Whittemore
Texas

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Rene
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Wednesday, August 10, 2011 9:13 AM

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All I said was it is no big deal to check the mags in flight. If you check them on the ground at 1750rpm, then there is no reason you couldn't check them in the air if you so desired. If the engine is beginning to lose power and after fuel, mixture, and carb heat has not helped, you need to check the mags. A mag can fail and while destroying itself begin to fire at the wrong time in the stroke. If it does this it may cancel out the power stroke. If an emergency arises and you check the mags and it runs fine on one, but loses performance on both, then select the one it runs fine on and elect to land as soon as possible. I believe the question was aimed at making sure the problem did not reocur once the aircraft was in flight. I had both mags fail in flight after 2 1/2 hours of flight. When the mags got hot the coils began to fail. When the rpm starts going down and everything you have tried is not working, then check the mags. They are independent of each other, but a failed mag that continues to fire, but at the wrong time,or has slipped its setting, is possible. My father had over 15,000 hours in Stearmans and this did happen. Without isolating the failed mag the engine will not make power.
I did not suggest going to the off position at any time once the engine is running as this will cause a backfire.

-------------------------
Rene St. Julien SRA 3882
15351 Navion Drive
Port St. Lucie, Florida 34987
Treasure Coast Airpark FL37
772 708 9210

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Tex
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Wednesday, August 10, 2011 9:29 AM

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Rene,

Guess I should have provided additional clarification. In an emergency situation, sure, as you stated you can (and should) check the mags. But to "test" them in the air as part of diagnosing a problem - bad idea. Again, just MHO.

-------------------------
Phil Whittemore
Texas

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kwallmueller
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Wednesday, August 10, 2011 6:34 PM

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Try checking the coil impeadance. this sounds just like a coil failure i had a couple of months ago.
Kelly

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Bones
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Wednesday, August 10, 2011 10:23 PM

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Hi Patricio
I have copied the following exerpt from Steve Curry at Radial engines inc (see below). Only applicable if you have VMN7DF mags. Also consider revarnishing the blocks - only after getting the moisture out ofcourse.


"Much of what was said about Lycoming magneto condensers and coils applies to the
SF7RN-1 and VMN7DF magnetos found on the W670 engine as well. They were just
not designed for a fifty year shelf life. Another factor with the W670 mags that
contributes to hard starting is weak rotating magnets. When originally built the rotating
magnet within the magneto was charged on a magnet charger (making it a four pole
permanent magnet). Unfortunately, the magnet can lose much of its magnetism over
time, thereby producing a weaker spark and making the engine harder to start. The
remedy is simply to have the magnet recharged.

Recently we have seen a rash of VMN7DF and VMN7DFA magnetos failing due to lack
of lubrication. The early symptom of this failure often is a hard to start engine. Originally
oil passages were provided in the magneto base for the front ball bearing and the plain
bushing on the large distributor gear, and in the coil cover for the rear ball bearing. In
service it was found that in order to sufficiently oil the distributor gear bushing, the ball
bearings were being over oiled. Bendix issued Service Bulletin #327 in February of 1955
instructing that the oil passages to the ball bearings were to be sealed off and that the ball
bearings were to be packed with magneto grease. The oil cup on the coil cover was to be
soldered closed and the oil cup on the magneto base was modified to oil only the
distributor large gear bushing. Since this passage continues past the bushing through the
back of the magneto, it is impossible to over oil the magneto. Excess oil simply drains
out the back of the mag. Bendix does not give an interval for oiling the magneto, but in
our experience, 25 hours is about as long as it should go without re-oiling. Without oil
the distributor gear becomes very hot and the magneto literally melts down. Remember,
it is impossible to over oil the magneto (if modified), but very easy to under oil it".

-------------------------
John Tabone SRA# 3535

Dee Why, Australia

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thammer
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Thursday, August 11, 2011 6:26 AM

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Phil, please explain why you think a mag check in flight is bad. From what I know a mag check in flight is no different or harder on the mags than a normal pre-takeoff mag check. A normal mag check does not include the 'OFF' position. That test should be done on shutdown.

Tye

-------------------------
thammer
N58712 PT-17 75-341

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boat
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Thursday, August 11, 2011 8:40 AM

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If an ohmmeter indicates continunity, you probably have a problem. I use a megger to check the coils. A megger is a test device that uses a high voltage of 500 or 1000 volts rather than the 1.5 typically used in an ohmmeter. The megger is a non-destructive tester that uses high voltage with very limited current. This device is typically used to measure high voltage leaks in electric motors, transformers, chokes, etc. Take your coil to just about any motor shop along with a picture of your plane, talk about Stearmans for a while and you can probably get your coil tested free of charge. The test takes only seconds once the tester is in hand.

It is not uncommon to register 1 or 2 meg ohms resistance on a good coil. The tech can give you guidance as to what level of resistance is acceptable.


-------------------------
Jim Stanfield

See my profile for contact information

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Tex
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Thursday, August 11, 2011 5:21 PM

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Tye,

We think of aircraft as having two sets of ignition systems (including spark plugs) for redundancy purposes, and certainly that is a major one. But it is also to provide a more efficient fuel burn which leads to greater horsepower (and spark plug life). We witness this affect when we do our mag checks and have a drop in rpm when we switch a mag off (there is ignition timing reasons for this as well). My new Ford truck has two spark plugs per cylinder, not for redundancy obviously, but for increased horsepower.

Just like we wouldn't check our fuel shutoff valve in flight, or pull our mixture back all the way in flight to test it, I can't see anything to be gained by checking the mags in flight. And I can imagine all kinds of things going wrong. If a defect exists in any component - the mag switch, the magnetos, the spark plugs or the wiring, we could possibly loose the use of one or more spark plugs, which might be the only ones firing on a given cylinder, or possibly loose use of the magneto itself. If this were to occur we are now flying on reduced horsepower, or worse yet a reduced number of cylinders.

We might think that's not really a problem, we could still land the airplane. And of course we know that will happen one way or another for sure. But if we need to execute a "Go Around" with diminished horsepower we might not clear the obstacle at the end of the runway, or get out of ground effect (especially on a hot summer day).

Seems to me the only thing we can't simulate on the ground when trying to diagnose what we think might be a mag related problem, is altitude.

We can certainly simulate rpm and load (we do that with partial rpm each time we do a run up, and full rpm and load when we check the adjustment of our propellor blades).

We can certainly simulate heat. We can do that with a heat gun, and in a very directed way, at each component individually, far more effective then flying.

We can check for problems with the wiring by ohm tests and visual inspection.

WARNING! THESE ARE BEST LEFT TO A MECHANIC DUE TO THE DANGER OF THE PROPELLOR (among other things): But some times electricity can creep out of a perfectly good looking (and tested) wire, or connections at the mag, or the spark plug, or anywhere else. One way to check for that is to take a sufficiently long piece of electrical wire, attach one end to a ground spot on the engine, the other to a long screw driver blade near the handle (alligator clips work good). Then, while the engine is running (stay away from the propellor!), and while holding on to the screw driver handle, run the tip of the screw driver along any ignition related wires and around any connections. If electricity is getting out it will jump to the screw driver blade and immediately go to ground (via the wire) and you will hear the difference in the way the engine sounds. We can check for intermittent internal connections by gently rapping on ignition components with something like a screw driver handle, or perhaps a piece of 1x2 wood. Anything that won't conduct electricity or damage the component. And by jiggling wires.

Moisture, and how to eliminate it has been covered in earlier posts so I won't go into that here.

So what about altitude? About the only thing I can envision that could be altitude related would be anywhere in the mag that sparks were jumping air gaps. Perhaps with the reduced air molecules and humidity at higher altitudes (do we really fly up there?) instead of jumping the gaps it would be easier for the electricity to crawl along the inside of the housing or cover, especially with moisture present. And once it has created a track to follow it get's easier and easier for it to do so. This would be easy enough to detect on the ground by looking for carbon tracing where the electricity crawled. Certainly a possibility, but seems like a major long shot to me (not the carbon tracing but it only happening at altitude), and this should always be checked for as part of any ignition system maintenance.

The above is just my humble opinion but I hope it is of value.



-------------------------
Phil Whittemore
Texas

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thammer
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Friday, August 12, 2011 6:23 AM

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Hi Phil, thanks for the explanation. I appreciate your opinion though I do not share it.

Tye

-------------------------
thammer
N58712 PT-17 75-341

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raisinfarmer
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Saturday, August 13, 2011 9:02 PM

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Oiling the mags. We have the SF7RN-1's. I don't see any way to oil this type. I don't see anything in the logs that address oiling. Are these packed? Response to this would be appreciated. No mag problems. Just wondering.
bill

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thammer
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Sunday, August 14, 2011 5:58 AM

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There are no oil holes on the SF7RN-1 mags. The bearings are greased at overhaul.

tye


-------------------------
thammer
N58712 PT-17 75-341

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