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How to bench-test old magneto?

 
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David Anderson
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PostPosted: Mon Jun 20, 2005 6:03 pm    Post subject: How to bench-test old magneto? Reply with quote

I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson
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Christopher Tidy
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PostPosted: Mon Jun 20, 2005 6:29 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

David Anderson wrote:
Quote:
I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson

When I've needed to test a magneto in the past I've just connected a
spark plug to it using a regular lead, then connected the outer metal of
the plug back to the metal case of the magneto. You can do this by
holding the plug against the magneto body instead, but it's an easy way
to give yourself a painful shock. Better to run a wire from a bolt in
one of the magneto mounting holes and clamp the other end against the
plug in a vice. Or use a jubilee clip to hold the wire against the plug.
Don't hold the magneto in the vice, or if you do use soft jaws and grip
it very gently. Then turn the magneto by hand (usually the coupling is a
pair of pins or suchlike which is easy enough to grip). Don't use your
VOM - the discharge from a magneto will likely break it. A weak spark
you can barely see but can usually hear. A good spark is bright and wide
and you'll have no problem seeing it. Sometimes a really good spark will
show a little 'cloud' of plasma around the plug tip.

Good luck,

Chris
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Christopher Tidy
Guest





PostPosted: Mon Jun 20, 2005 6:35 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

jim rozen wrote:
Quote:
In article <UnDte.392$W74.68@newssvr30.news.prodigy.com>, David Anderson says...

I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson


First off test to see if the coil is continous. This probably
looks like a telephone mag, right? It has the horse-shoe magnets
over the top?

Sorry, perhaps this is how you intended to use your VOM? This is fine. I
thought you meant using the VOM to measure the magneto's output voltage!

By the way, have you tried fitting a new spark plug? It's worth a try
before you start testing the megneto.

Chris
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Christopher Tidy
Guest





PostPosted: Mon Jun 20, 2005 6:39 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

jim rozen wrote:
Quote:
In article <UnDte.392$W74.68@newssvr30.news.prodigy.com>, David Anderson says...

I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson


First off test to see if the coil is continous. This probably
looks like a telephone mag, right? It has the horse-shoe magnets
over the top?

Sorry, perhaps this is how you intended to use your VOM? This is fine. I
thought you meant using the VOM to measure the magneto's output voltage!

By the way, have you tried fitting a new spark plug? It's worth a try
before you start testing the megneto.

Chris
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Christopher Tidy
Guest





PostPosted: Mon Jun 20, 2005 6:48 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

Christopher Tidy wrote:
Quote:
David Anderson wrote:

I own a vintage International Harvester single-cylinder hopper cooled
farm engine in mechanically excellent condition. I believe the model
designation is <LB>, and probably dates from the late 1920's or
30's. It used to start on the first crank, but no more. I suspect
the magneto is weak. Any suggestions how to bench-test this mag? I
have a professional quality digital VOM but no specialized magneto
tools. The mag has a centrifugal device that "snaps" at slow speeds.
Regards, Dave Anderson


When I've needed to test a magneto in the past I've just connected a
spark plug to it using a regular lead, then connected the outer metal of
the plug back to the metal case of the magneto. You can do this by
holding the plug against the magneto body instead, but it's an easy way
to give yourself a painful shock. Better to run a wire from a bolt in
one of the magneto mounting holes and clamp the other end against the
plug in a vice. Or use a jubilee clip to hold the wire against the plug.
Don't hold the magneto in the vice, or if you do use soft jaws and grip
it very gently. Then turn the magneto by hand (usually the coupling is a
pair of pins or suchlike which is easy enough to grip). Don't use your
VOM - the discharge from a magneto will likely break it. A weak spark
you can barely see but can usually hear. A good spark is bright and wide
and you'll have no problem seeing it. Sometimes a really good spark will
show a little 'cloud' of plasma around the plug tip.

Of course in your case you don't need to remove the magneto from the
engine to perform the test above. I'm being dense tonight...:-D

Chris
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F. George McDuffee
Guest





PostPosted: Mon Jun 20, 2005 7:02 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

see:
http://www.lindsaybks.com/dgjp/djgbk/magneto/index.html

-------
On Mon, 20 Jun 2005 17:52:52 GMT, "David Anderson"
<fdavidander@sbcglobal.net> wrote:

Quote:
I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson
Back to top
jim rozen
Guest





PostPosted: Mon Jun 20, 2005 7:02 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

In article <UnDte.392$W74.68@newssvr30.news.prodigy.com>, David Anderson says...
Quote:

I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson

First off test to see if the coil is continous. This probably
looks like a telephone mag, right? It has the horse-shoe magnets
over the top?

Jim


--
==================================================
please reply to:
JRR(zero) at pkmfgvm4 (dot) vnet (dot) ibm (dot) com
==================================================
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Steve Peterson
Guest





PostPosted: Mon Jun 20, 2005 7:03 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

"F. George McDuffee" <gmcduffee@fpc.cc.tx.us> wrote in message
news:592eb1pms3nu8er2jcd2fqo94eva5jekb1@4ax.com...
Quote:
see:
http://www.lindsaybks.com/dgjp/djgbk/magneto/index.html

Ha ha. I like the last paragraph in the magneto link.
Steve
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Eric R Snow
Guest





PostPosted: Mon Jun 20, 2005 9:03 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

On Mon, 20 Jun 2005 18:48:30 +0000 (UTC), Christopher Tidy
<cdt22NOSPAM@cantabgold.net> wrote:

Quote:
Christopher Tidy wrote:
David Anderson wrote:

I own a vintage International Harvester single-cylinder hopper cooled
farm engine in mechanically excellent condition. I believe the model
designation is <LB>, and probably dates from the late 1920's or
30's. It used to start on the first crank, but no more. I suspect
the magneto is weak. Any suggestions how to bench-test this mag? I
have a professional quality digital VOM but no specialized magneto
tools. The mag has a centrifugal device that "snaps" at slow speeds.
Regards, Dave Anderson


When I've needed to test a magneto in the past I've just connected a
spark plug to it using a regular lead, then connected the outer metal of
the plug back to the metal case of the magneto. You can do this by
holding the plug against the magneto body instead, but it's an easy way
to give yourself a painful shock. Better to run a wire from a bolt in
one of the magneto mounting holes and clamp the other end against the
plug in a vice. Or use a jubilee clip to hold the wire against the plug.
Don't hold the magneto in the vice, or if you do use soft jaws and grip
it very gently. Then turn the magneto by hand (usually the coupling is a
pair of pins or suchlike which is easy enough to grip). Don't use your
VOM - the discharge from a magneto will likely break it. A weak spark
you can barely see but can usually hear. A good spark is bright and wide
and you'll have no problem seeing it. Sometimes a really good spark will
show a little 'cloud' of plasma around the plug tip.

Of course in your case you don't need to remove the magneto from the
engine to perform the test above. I'm being dense tonight...:-D

Chris
After you have checked the points and condenser and tested the coil

for continuity if that doesn't solve the problem the this how I test
'em and sometimes fix 'em: First, Be really sure the condenser isn't
shorted or open and the points are clean and adjusted. Then see if you
get spark. If nothing happens try a different spark plug. If nothing
happens then stick a screw driver in the plug cap and grab it. If it
just has a bare end then grab that. Spin the mag and see if you get a
tingle. If you do then it may be that moisture is in the coils. Bake
the mag in a 250 degree oven for an hour or so and let the mag cool
off in the oven. Test for spark again. Don't use your hand this time
though. The first time I fixed a Fairbanks Morse mag this way I was
dubious and grabbed the bare wire and spun the mag. BIG mistake. It
really gave my arm a jolt. If you can get the coil out of the housing
easily you may want to do that. I bake the whole thing but it might
hurt yours. Check it while baking to make sure nothing is melting. And
if you really want sparks then bake in a microwave. It won't do the
mag much good but it will spark while the oven is on.
ERS
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Christopher Tidy
Guest





PostPosted: Mon Jun 20, 2005 9:51 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

Quote:
I own a vintage International Harvester single-cylinder hopper cooled
farm engine in mechanically excellent condition. I believe the model
designation is <LB>, and probably dates from the late 1920's or
30's. It used to start on the first crank, but no more. I suspect
the magneto is weak. Any suggestions how to bench-test this mag? I
have a professional quality digital VOM but no specialized magneto
tools. The mag has a centrifugal device that "snaps" at slow speeds.
Regards, Dave Anderson


When I've needed to test a magneto in the past I've just connected a
spark plug to it using a regular lead, then connected the outer metal of
the plug back to the metal case of the magneto. You can do this by
holding the plug against the magneto body instead, but it's an easy way
to give yourself a painful shock. Better to run a wire from a bolt in
one of the magneto mounting holes and clamp the other end against the
plug in a vice. Or use a jubilee clip to hold the wire against the plug.
Don't hold the magneto in the vice, or if you do use soft jaws and grip
it very gently. Then turn the magneto by hand (usually the coupling is a
pair of pins or suchlike which is easy enough to grip). Don't use your
VOM - the discharge from a magneto will likely break it. A weak spark
you can barely see but can usually hear. A good spark is bright and wide
and you'll have no problem seeing it. Sometimes a really good spark will
show a little 'cloud' of plasma around the plug tip.

Of course in your case you don't need to remove the magneto from the
engine to perform the test above. I'm being dense tonight...:-D

Chris

After you have checked the points and condenser and tested the coil
for continuity if that doesn't solve the problem the this how I test
'em and sometimes fix 'em: First, Be really sure the condenser isn't
shorted or open and the points are clean and adjusted. Then see if you
get spark. If nothing happens try a different spark plug. If nothing
happens then stick a screw driver in the plug cap and grab it. If it
just has a bare end then grab that. Spin the mag and see if you get a
tingle. If you do then it may be that moisture is in the coils. Bake
the mag in a 250 degree oven for an hour or so and let the mag cool
off in the oven.

A friend told me that where he works they put faulty electric motors in
the oven for a few hours to dry them out. If they still don't work, they
spray a load of varnish all over the windings and bake the motor again.
If it still doesn't work they get a rewind.

Chris
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gfulton
Guest





PostPosted: Mon Jun 20, 2005 11:02 pm    Post subject: Re: How to bench-test old magneto? Reply with quote

"Eric R Snow" <etpm@whidbey.com> wrote in message
news:vu9eb1hof3mk2qbou2q21c77kqur9k9fr9@4ax.com...
Quote:
On Mon, 20 Jun 2005 18:48:30 +0000 (UTC), Christopher Tidy
cdt22NOSPAM@cantabgold.net> wrote:

Christopher Tidy wrote:
David Anderson wrote:

I own a vintage International Harvester single-cylinder hopper cooled
farm engine in mechanically excellent condition. I believe the model
designation is <LB>, and probably dates from the late 1920's or
30's. It used to start on the first crank, but no more. I suspect
the magneto is weak. Any suggestions how to bench-test this mag? I
have a professional quality digital VOM but no specialized magneto
tools. The mag has a centrifugal device that "snaps" at slow speeds.
Regards, Dave Anderson


If you need to get it remagnetized, see if there's a speedometer repair shop
in your area. They will likely have the remagnetizing rig shown on the
Lindsay book link someone posted. He should check field polarity with a
compass before he puts the mag. housing between the magnets and turns on the
current.

Garrett Fulton
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Jordan
Guest





PostPosted: Tue Jun 21, 2005 12:03 am    Post subject: Re: How to bench-test old magneto? Reply with quote

Could be dirty points, if it used to start well recently?
Check also the body of the mag is grounded to vehicle chassis, either by
it's normal clamping arrangement or with earthing wire.
To test, run it for an hour or so (to get it to normal operating
temperature) with a rig-up to an electric motor. To simulate operating
conditions (engine compression), provide a gap of say 3/16 inch - not
too much or you could cause breakdown of the insulation within magneto.
It doesn't pass the test if more than 5% spark is missing.

Jordan


Quote:
It used to start
on the first crank, but no more. I suspect the magneto is weak.
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David Billington
Guest





PostPosted: Tue Jun 21, 2005 1:02 am    Post subject: Re: How to bench-test old magneto? Reply with quote

Sounds like a 7hp JAP industrial engine engine I had, hand crank start
and normally went on the 2nd go but if you left it standing for a year
then it required the points cleaning or it wouldn't start. After
cleaning the points then starting was again 2nd go.

David Anderson wrote:

Quote:
I own a vintage International Harvester single-cylinder hopper cooled farm
engine in mechanically excellent condition. I believe the model designation
is <LB>, and probably dates from the late 1920's or 30's. It used to start
on the first crank, but no more. I suspect the magneto is weak. Any
suggestions how to bench-test this mag? I have a professional quality
digital VOM but no specialized magneto tools. The mag has a centrifugal
device that "snaps" at slow speeds. Regards, Dave Anderson

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Guest






PostPosted: Tue Jun 21, 2005 1:02 am    Post subject: Re: How to bench-test old magneto? Reply with quote

I wonder if that old mag has a bad kill switch, or a shorted
kill wire. Mags are shut off by grounding the primary lead (the
connection between the coil and points) and if the switch is stuck or
the wire chafed against ground it will be dead.
Points gap in a mag is critical, since it controls the E-gap,
or the point in the magnet's rotation at which the points open.
Aircraft mags are set using various protractor tools or other means of
positioning the rotor for points opening; much more accurate than gap
setting and gives the hottest spark.
Bad capacitors are common, and I would expect a mag as old as
that to have cap problems. Without the capacitor it won't fire at all.
Lots of textbooks and mechanics teachers wil tell you that it only
prevents the points burning, but just try to run without it! It's
function is to absorb the coil surge as the points open to prevent
arcing; arcing both burns the points and prevents the sudden current
shutoff we need for rapid field collapse and hot spark.

Dan
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