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John Deere Tractors Discussion Forum
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1940 B Magneto Tune-Up

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B_Bob

03-23-2008 06:58:21




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Have a decent running engine but want to give the Magneto a general tune-up. Has good spark, new plugs and wires and timing is good with flywheel, have adjusted valves and rebuilt carb. Other than replacing points, condensor and gaskets, is there anything I should or shouldn't check or do more in depth while I've got it open? Have a good working knowledge of Mags but no repair experience and the JD-4 manual just covers timing. Thanks,
Bob

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36 coupe

03-24-2008 03:08:28




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 Re: 1940 B Magneto Tune-Up in reply to B_Bob, 03-23-2008 06:58:21  
Mag parts are expensive.If it has good spark I would leave it alone.



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Duane Larson

03-23-2008 18:11:43




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 Re: 1940 B Magneto Tune-Up in reply to B_Bob, 03-23-2008 06:58:21  
B_Bob,
What kind of magneto do you have on your tractor? Is it a Wico C or X, or something else?



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B_Bob

03-23-2008 18:48:05




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 Re: 1940 B Magneto Tune-Up in reply to Duane Larson, 03-23-2008 18:11:43  
Maybe I'll get it complete this time. WICO Type "X".



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B_Bob

03-23-2008 18:44:25




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 Re: 1940 B Magneto Tune-Up in reply to Duane Larson, 03-23-2008 18:11:43  
The Mag is Model XH 1042.
Thanks



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Duane Larson

03-24-2008 20:14:14




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 Re: 1940 B Magneto Tune-Up in reply to B_Bob, 03-23-2008 18:44:25  
B_Bob,
With all the work you have done, about all I could suggest would be to make sure the edge gap is correct. Looking at the left hand side of the magneto, under the coil, when the trailing edge of the rotating magnet has left the pole face by a distance of 0.095", the points should just be opening. It is at that instant the maximum high voltage is being generated in the coil. This distance is referred to as the Edge Gap, and is covered in SM-2029, the electrical service manual. Make sure the clips holding your coil in place are not cracked around where the screws go through them - they are prone to cracking and breaking and when that happens it is easy for a piece of the clip to be attracted to the magnet and gouge the pole face. Finally, check the wear on the drive cup at the point where it contacts the driven flange - wear at that point, and where the driven flange slides on the magnetic rotor shaft - both cause the timing to advance - eg, from 25 deg BTDC to 30 deg.

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