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How do you test a Model B Magneto?

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Prune Picker

10-22-2001 16:34:30




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My 1944 BN always starts right up and runs like a watch, except recently. It is becoming progressively harder to start (electric start with a 12V battery). Plugs look good but have a some carbon fouling. Spark is visable but not particularly bright. How can I check the mag without removal and sending it to a shop, where it will undoubtably need an "overhaul"?




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John T

10-23-2001 11:17:43




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 Re: How do you test a Model B Magneto? in reply to Prune Picker, 10-22-2001 16:34:30  
Prune, For no more than they cost, I would first try a fresh new set of plugs (Autolite 386 or 388 I think for that tractor) and see if she starts any better, then do the points and condensor (how do the wires look?) as Clooney suggested, and if you do need a new coil, I reccomend a HOT coil which you can buy from Robert Beavers. Good Luck Ol John T



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Clooney

10-22-2001 17:00:13




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 Re: How do you test a Model B Magneto? in reply to Prune Picker, 10-22-2001 16:34:30  
Prune Picker, what mag does your B have on it? Those things are hard to test at home. First does the impulse have a good snap when the engine is turned over? If the impulse is snapping good you should probably remove any internal rust on the rotor & coil core then install a new set of points & condenser & see if the you have a good blue 3/16"+ spark if so that's about all you can do, if not install a new coil & if that doesn't get you a good blue spark & it isn't a Wico X have the rotor re-magnetized, if it is an X the rotor shouldn't need any attention.

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Prune Picker

10-23-2001 17:50:25




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 Re: Re: How do you test a Model B Magneto? in reply to Clooney, 10-22-2001 17:00:13  
It's a Wico X. Snaps well. Rotor looks worn as does points of contact in cap. Points look fine. I don't have a 3/16" blue spark.... smaller and not as bright. Plug wires are new. Plugs are Champions bought at the Deer dealership. Thanks for the advice.



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Clooney

10-23-2001 18:52:56




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 Re: Re: Re: How do you test a Model B Magneto? in reply to Prune Picker, 10-23-2001 17:50:25  
Prune Picker, sounds like you have a weak spark. A bad condenser as well as a weak coil can cause a weak spark. In the X, worn bushings, excessive rotor air gap (not cap rotor but rotating magnet rotor), rust on the coil core & rotor (magnet rotor) can also cause a weak spark. It might be a good idea to clean the inside of rust then put in a new condenser & see where you are at & if finances allow install a new coil while you are in there. Once in a while I will disagree with my old friend John T. & the high output coil is one I disagree on. I believe unless you are running high compression, higher than stock RPM, running large spark plug gaps or building a puller there is no gain to be had from a high output coil as the spark will only build as high as the system resistance & plug gap allows & a stock coil is plenty for that & about $15.00 cheaper. There is nothing wrong with using the high output coil & it will work fine I just don't think it is worth the extra money on a stock engine. The X doesn't seem to have a problem with the rotor loosing it's magnetism like the older mags but I suppose given the age of the mag anything is possible.

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