wd magneto
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=152831
Printed Date: 20 Aug 2025 at 7:56am
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: wd magneto
Posted By: Snowvalley Dairy
Subject: wd magneto
Date Posted: 07 Aug 2018 at 10:09pm
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i'm working on a magneto for a friend, its a fairbank morris fmj. The tractor ran last fall. It has no spark, and turns over very easily. with one finger it will turn over several times. I've never worked on a mag but, they should have considerable resistance, shouldn't they?. what should i look for?
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Replies:
Posted By: Steve in NJ
Date Posted: 08 Aug 2018 at 5:18am
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Not sure if you mean the engine turns over easily, or If the Mag turns over easily. If the Mag turns over easily, it probably has a broken impulse spring. You should have resistance when you go to turn the impulse. Another problem could be the pawls are worn, and they're not catching the impulse lock pin in order to "load" the impulse spring. When you turn the impulse, it should lock, and the spring should give you resistance as you wind it up... HTH Steve@B&B
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39'RC, 43'WC, 48'B, 49'G, 50'WF, 65 Big 10, 67'B-110, 75'716H, 2-620's, & a Motorhead wife
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Posted By: B26240
Date Posted: 08 Aug 2018 at 6:03am
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There was a video on here a couple of days ago by Rachael on how to rebuild a mag, watch that to see if you think you are qualified to do the rebuild. Magnetos consist of several components that ALL have to work. Otherwise send it to a rebuilder such as Steve.
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Posted By: DaveKamp
Date Posted: 08 Aug 2018 at 7:28am
Yeah... the coupling will move around fairly easily, but when you're turning it the forward direction, the impulser coupling will engage and wind up. If you're gutsy enough to keep turning, you'll wind it up, and really gutsy enough to hold it when that impulser triggers, it'll release, making the unit jump in your hands, and it'll zap the crap out'a ya, after which you'll drop it to the floor, and probably smash the fragile and expensive phenolic distributor cap end... so hold it over a well-padded sofa if you're gonna do that, okay?
Magnetos are easy to rebuild and setup if you have the RIGHT tools. A magneto test set... with varible speed... is the first piece. It has a mount that the mag clamps to, and a bucket full of adapters to couple the tester's drive to a myriad of magnetos. It has a row of spark plugs with wires to plug into the mag, and a speed control on the motor. The speed control is necessary because one must verify that the impulser coupling activates below a certain speed, and locks out above that speed.
The other stuff one needs... is a big batch of spare parts, a condenser test set, a box or three of extra tungsten contacts, a bead blaster, a variety of cleaning supplies, lots of books... extra screws, nuts, and bolts, thread chasers, a vertical mill (to remove broken off screws) and a small lathe for cleaning up worn shafts, making replacement bushings... felt and gasket material, a little paint, a coil-winding jig, some spools of enamel HV wire... a pressure pot to force epoxy into a new coil, a magnet recharger...
That's if you wanna do a 'real' rebuild. If you just wanna open it up and shove points and condenser in it, you can... but after seeing what my magneto guy goes through to make mine so darned good... I'd recommend carrying it to The Master, and writing a check.
It's kinda like fixing a clock...
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Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest.
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Posted By: DougS
Date Posted: 08 Aug 2018 at 7:41am
What B26240 said. Be aware that Rachel had trouble disassembling parts of the mag. Be careful of the armature too. You don't just put it in a press and have at it. Here's a start:
http://www.oldcroak.com/fmj-series-1942-bulletin-2856a/" rel="nofollow - http://www.oldcroak.com/fmj-series-1942-bulletin-2856a/
Rachel does give some good pointers, but note that she had to give up on one of the magnetos. Strip a screw hole or break off a screw head and you've made the next guy's job much more difficult.
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Posted By: Gerald J.
Date Posted: 08 Aug 2018 at 10:21am
I have a shop manual for the FM magneto on line at:
http://geraldj.networkiowa.com/Trees/FM-Xmagneto.pdf" rel="nofollow - http://geraldj.networkiowa.com/Trees/FM-Xmagneto.pdf
Gerald J.
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Posted By: HoughMade
Date Posted: 08 Aug 2018 at 10:58am
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They've covered the impulse above, but if spinning backwards there is little to no resistance and when you put a screwdriver by the magnets on the armature there is not a strong pull, the magnets may be weak.
If everything else is in perfect condition, a mag will make spark with weak magnets leading some to think that high tension mag magnets never get weak. These people are wrong. They can get weak over 50+ years (in my case 67). However, if you have weak, no or intermittent spark, chances are that the magnets will never be the only problem because they are the last thing to go bad.
If the magnets are strong, it will (obviously) make better spark and even make strong spark when everything else (points, condenser, coil) are operable, but less than perfect. If they are extremely weak, little to no spark, but by that time, you probably have a bad coil, points or condenser as well.
Then, after having changed the easily replaceable parts and it still not starting great, people do things like convert to 12 volts or install a battery ignition in an effort to solve what a $15-20 mag recharge, in addition to a good coil, points and condenser, can do. Now that my mag is right, my original 6 volt positive ground starter cranks for under 2 seconds to start the tractor. Trying to figure out why I would want to improve on that.
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1951 B
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