Farm Equipment - Magneto question

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Magneto question

Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=20592
Printed Date: 10 Feb 2025 at 7:30pm
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Topic: Magneto question

Posted By: TMiller/NC
Subject: Magneto question
Date Posted: 09 Nov 2010 at 5:03pm

Will a Fairbanks Morse FM 4B magneto work on a 39B.

Replies:

Posted By: morton(pa)
Date Posted: 09 Nov 2010 at 5:39pm

I believe you mean FMJ4B. Yes, they were the stock magneto that came on the B's. The FMX replaced the FMJ in the 50's. I was told that the FMX never came from the factory, but was instead a dealer shelf part swap if the FMJ went bad.

Posted By: tododell48
Date Posted: 09 Nov 2010 at 6:21pm

Yes the fmj4b will deinitely work.

Posted By: wjohn
Date Posted: 09 Nov 2010 at 7:41pm

If it is just an FM 4B and not an FMJ 4B then I'm not sure if it will work or not - they are two different models. I think I've seen the FM 4B mags on WC's before, but I don't know about B's. Hopefully someone else with more experience will chime in.

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1939 B, 1940 B, 1941 WC, 1951 WD, 1952 CA, 1956 WD-45

Posted By: TMiller/NC
Date Posted: 09 Nov 2010 at 8:34pm

I've looked again fellows and there isn't any J or X just FM 4B. Tim

Posted By: GlenninPA
Date Posted: 09 Nov 2010 at 8:54pm

Check the lag angle set on the FM 4B as opposed to the FMJ 4B. I believe, if my memory serves me right, (which it doesn't sometimes) that you need a 30 degree lag angle for the spark advance to make the 125 engine run at it's best.

Posted By: TMiller/NC
Date Posted: 09 Nov 2010 at 9:15pm

The reason for all the question is........ this magneto was on the tractor when I bought it. Have not had the engine run, but the coil appears to be bad in the mag. If it isn't correct for the B engine I'll look for another rather than have this one repaired. Thanks Tim