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A/C B Magneto Problem

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Gary Davis

11-21-2003 20:22:30

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Help! I'm working on na A/C B it has a Fairbanks Morse FM Magneto. It came in to my lawn mower shop not running. I've worked on 1 cyl wisc. mags but not 4 cyl.s The owner said sometimes if you cranked it long enough it would start. If it quit though you might have to wait till the next morning to start again. I put a set of points and condensor in the mag and it started right up. I ran it for 15 min and turned it off for 15 min. and then it would not start again. I checked for spark and it had a paper thin gap it would jump.I took the cap off the mag. and checked the point gap- OK but where I got lost was that the coil seemed to be grounded wether the points were open or closed.My question is- If I disconnect the wire coming from the coil to the points, with a continuity tester hooked to that wire and ground the other tester wire end to the mag frame will it show continuity or will it be open? Thanks Gary Davis

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Joseph Tucci Jr.

12-01-2003 19:08:26

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 Re: A/C B Magneto Problem in reply to Gary Davis, 11-21-2003 20:22:30  
I would say no I have a 47 AC c same mag I got her in 500 parts after rebuilding the moter I got almost no spark From mag best bet is to change over to 12 V add a coil use points in mag run hi voltage wire in to mag remove org. coil rout hi volt to coiltower make a distneto!!! any ? call 724-758-1006 or Email josephjttop@aol.com

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Steve in N.J.

11-23-2003 07:36:11

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 Re: A/C B Magneto Problem in reply to Gary Davis, 11-21-2003 20:22:30  
As Glen mentioned, if the machine is equipped with a kill switch, disconnect that first. Many times the coil gets the blame and it isn't the coil at all. Another thing to check is the kill circuit which runs from the contact set to the stud that comes out the side of the mag. If the machine isn't equipped with a kill switch in the Ammeter panel, then it should have a shut down lever mounted off the stud. We rebuild, have parts, and service F/M mags and on occasions I'll find the jacket that covers the circuit kinks and splits which opens the circuit to grounding on the housing. If you do suspect the coil, another problem child F/M had was in their early units, they used a taped/tar coil. These coils will eventually "leak" and as they warm up, start arching to the inside of the housing causing the engine to shut down till it cools off. We keep epoxy style coils in stock and automatically replace those old tar coils on a rebuild. Since you did replace the condensor, it would be very unlikely that the new condensor you installed would be bad. If you have any more troubles, drop us a line, I'd be happy to help ya out.. Good Luck!
Steve/B&B Custom Circuits customcircuits@aol.com

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Glen in TX

11-22-2003 09:44:40

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 Re: A/C B Magneto Problem in reply to Gary Davis, 11-21-2003 20:22:30  
You will still have continuity. If you have a coil tester see if it gives test specs. for this coil. You will have to have most mag coils out of the housing to check them properly with a coil tester. Is is just a FM or FM-J, FM-X or other? If it runs till warmed up and then shuts off and doesn't have spark, you have a coil problem. As the coil warms and expands the wiring and insulation inside expands and a wire is probably pulling apart or insulation is breaking down. Test or change the condenser again too to rule it out. Make sure there isn't a break in the outside coil case or coil wire. Disconnect any kill switches & wiring also and test them for problems.

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David Bacon

11-22-2003 21:00:51

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 Re: Re: A/C B Magneto Problem in reply to Glen in TX, 11-22-2003 09:44:40  
Gary done hit it ground zero---it is the coil Dave NE Iowa

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