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| | |-+  Testing the coils on a twin spark Fairbanks Morse Magneto
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Author Topic: Testing the coils on a twin spark Fairbanks Morse Magneto  (Read 2482 times)
barry100
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« on: February 14, 2012, 07:06:17 am »

Hi All
I have got 4 magnetos all for same engine painted army green ,  3 Fairbanks and one Wyco  all Twin spark (  no dissy both fire together) and none work, Coil test: Primary to core ( earth) about 3 omhs.  Secondary ( high tension spring) to core , infinity, open, ,  between the two high tension  springs 12000 omhs ,all look clean and good condtion inside,  anyone know what the readings  mean
Regards Barry
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Thob
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« Reply #1 on: February 15, 2012, 02:24:37 am »

Well - since no one else has replied, I put my ignorance out for everyone to see.  I'm no expert on magnetos, but those sound like reasonable numbers to me.  Do these magnetos have points and condenser?  I'd look there for trouble - make sure the points read zero ohms when installed.  Make sure the condenser is good, if you have a way to test it.  I think it needs to be good for several hundred volts (I had one on car engine years ago that measured the correct uF but had a voltage breakdown of only about 15 volts.  I didn't know then that it needed to be much more).

When I think of Listers, I usually think of diesels.  You might find more responses on these forums:

http://www.microcogen.info/index.php

http://www.smokstak.com/forum/forumdisplay.php?f=3


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Witte 98RC Gas burner - Kubota D600 w/ST7.5KW head.
I'm not afraid to take anything apart.
I am sometimes afraid I'm not going to get it back together.
Thob
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« Reply #2 on: February 15, 2012, 02:27:16 am »

One more thing - are they still magnetized?  They CAN become demagnetized.

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Witte 98RC Gas burner - Kubota D600 w/ST7.5KW head.
I'm not afraid to take anything apart.
I am sometimes afraid I'm not going to get it back together.
Thob
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« Reply #3 on: February 15, 2012, 02:54:43 am »

Some information here might be of use (repair instructions for Fairbanks Morse Magneto):

http://www.oldengine.org/members/diesel/Magnetos/MagFmBook.htm

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Witte 98RC Gas burner - Kubota D600 w/ST7.5KW head.
I'm not afraid to take anything apart.
I am sometimes afraid I'm not going to get it back together.
barry100
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« Reply #4 on: February 16, 2012, 06:30:12 am »

Hi Thob
Thanks for the tips, I had tested the points and condeser both ok, when I operate the magneto(s) I get a spark at the points indicating primary side ok ( and magnet?), normaly with single spark , the reading is to H/T to earth, don't know about the twin spark though
Regards  Barry
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Thob
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« Reply #5 on: February 16, 2012, 03:14:20 pm »

Again, I'll contribute my ignorance of magnetos, but on modern engines the high tension coil often serves two cylinders, one end of the coil goes to each spark plug.  No ground on the high tension side, one cylinder is on the exhaust stroke and one cylinder on the compression.  The spark plug on the exhaust stroke side is easy to fire (relatively low voltage to arc across), so it doesn't require as much voltage as the spark plug under compression.  So you only need like 1.2 or 1.4 times the voltage of a single cylinder to fire two cylinders.  One revolution later the roles of the cylinders are reversed.  This is typically used on 4 cylinder engines with two coils, or 6 cylinder engines with 3 coils.

Because both spark plugs are in series with the coil, you need them both connected to get a spark.
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Witte 98RC Gas burner - Kubota D600 w/ST7.5KW head.
I'm not afraid to take anything apart.
I am sometimes afraid I'm not going to get it back together.
barry100
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« Reply #6 on: February 18, 2012, 12:19:31 am »

Hi Thob
Thanks for the tips, I was thinking because of the two H/T s  not grounding to the the core was OK ? , but not realising I would have to test two plugs together to get an earth for one of the plugs , will test ASAP, I did think it was strange they all look clean and in good condition , but none working
Regards Barry
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barry100
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« Reply #7 on: February 18, 2012, 07:17:15 am »

Hi Thob
They work, I put a plug on each lead and got a spark on both, I bet a lot of magnetos have been scraped in the past thinking they were U/S, the only reason I thought they were OK because I had four all good inside , it would have been unusual for all to be US
Thank again
Barry
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Thob
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« Reply #8 on: February 18, 2012, 02:10:48 pm »

You're welcome - maybe I'm not so ignorant after all  Shocked

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Witte 98RC Gas burner - Kubota D600 w/ST7.5KW head.
I'm not afraid to take anything apart.
I am sometimes afraid I'm not going to get it back together.
buickanddeere
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« Reply #9 on: February 19, 2012, 01:58:12 pm »

   Is there a tab or button to short out the mag so the engine can be shut down?
   Coils sound about right. There should be no path between the primary and secondary windings on those.
    Are the high voltage wires solid core?
     I've seen those old magneto magnets boosted by milling a slot and inlaying those super high strength ceramic magnets. 
     
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