How to bench-test old magneto?

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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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

jim rozen wrote:
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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

jim rozen wrote:
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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

see: http://www.lindsaybks.com/dgjp/djgbk/magneto/index.html
------- On Mon, 20 Jun 2005 17:52:52 GMT, "David Anderson"
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Ha ha. I like the last paragraph in the magneto link. Steve

David Anderson wrote:
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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

Christopher Tidy wrote:
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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

On Mon, 20 Jun 2005 18:48:30 +0000 (UTC), Christopher Tidy
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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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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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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

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

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:
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