John Deere A magneto

MikeinInd · Feb 22, 2013

I have a 1943 John Deere model A I have been tinkering with. Its never been easy to start and never ran quite as well as it should. Would run uneven at times but didnt sound like an all out miss. It sat for quite some time before I got it so I figured the carb would be suspect #1 but its very clean inside and all the passage ways are open. Suspect #2 was of course the mag. Cleaned the points and checked the gap. All seemed fine but when I tested it by turning it over by hand I noticed that spark seemed to be weaker on the lower socket than the top and it would sometimes fail to fire when it should have. I hooked up an adjustable spark tester and sure enough the top will throw a stronger spark than the bottem but will also miss here and there with the tester. By far no expert on mags but I would think that spark would be the same on both sockets weak or strong. Both sockets and all contacts inside the cap are clean.

D beatty · Feb 22, 2013

I would get a new cap and rotor.

John B. · Feb 23, 2013

Try the cheapest things first. Sparkplugs, points and condenser. Then I would worry about the magneto itself. I had a Farmall H with a distributor not a magneto, that would give trouble starting at times. I would clean the points and everything would work like it should. One time it wouldn't start and left me stranded. I called a tow company to get it home for me. I bought a new set of points and condenser. It started and has run fine since by just putting the new points in. I experimented by not replacing the condenser and sure enough it was the points. I did keep the condenser to replace at a later time. Let us know what you find.

John T · Feb 23, 2013

Its the same points and condensor and rotor magnet and impulse spring etc for BOTH sparks so Id suspect a cap or rotor or maybe the bushing/bearing or cam is bad and the points gap is different on one lobe versus the other. My "usual suspects" are points, then condensor, then cap n rotor then rotor magnet strength etc.

John T

Ken L · Feb 23, 2013

Had a 46 A that started to miss on the right cyl. Checked everything a couple times. Plugs, points and condenser weren't very old. Couldn't find the problem but I knew it was the mag. Took it off and went to the repair shop where they put it on a test stand. When he got done, he smiled and said that I could have saved a lot of money if I had just replaced the points. The whole bill was less than $10.00.

red.green · Feb 23, 2013

What you didn’t state in your description was how you hooked up your spark tester. Did you hook it to the existing spark plug wires, if so you just proved one wire not firing as well as the other, replace both. If you used the same test wire to test both, than you have eliminated the wire as a cause. Next clean and check the plug wire holes on the cap, frequently there are dirty.

As John T stated you have to look at the parts relating to only one side firing. Check the armature bushing for excessive wear, as a wobble will create a misfire. Check the rotor for any slop or play on the shaft, lastly I would swap out the condenser, as a weak one sometimes fail on the second spark. I hope this helps.

Earl-IL · Feb 23, 2013

I do a lot of magneto repair. We could help you more if we new what magneto model & manufacture you have on your tractor.

MikeinInd · Feb 23, 2013

Ok, to update this is a Wico X mag. I used a known good plug wire from an old garden tractor to do all the testing. I traded the condenser out for a new one I have with the same results. There is no slop at all in any of the rotating parts.

John B. · Feb 23, 2013

Make sure your spark plug wires have wire in them not the carbon fiber. Carbon fiber wires also known as TVRS (television & radio suppressor) wires will not work with a magneto.

Earl-IL · Feb 23, 2013

You said you cleaned the points? Point contact finish needs to be as good as you can make it. The 2 surface on the points need to have a mirror finish,not a filed or coarse sand paper finish.Check this surface with a magnify glass. Polish with 1000 grit. Some of the new points we get today are miss aligned and poor finish. check the spring terminal in the cap for corrosion. Pay close attention to the screws in the rotor are not sticking up higher than the carbon in the middle of the rotor and rubbing on the cap to rotor contact. Is the rotor tight on the shaft? It can't flop around. I have had the point pin worn to where the points will slide ,before they break.This throws the timing to the edge gap off. I have new pins. Some times you will need to gap points to .016 -- .017. If your mag has a kill button disconnect & test.

Earl-IL · Feb 23, 2013

knew,know spell check Earl ?

MikeinInd · Feb 23, 2013

The points are as clean as I could get them, not a mirror finish but not a scratchy, dull look either. I used 800 grit paper and a dremel tool with a small brush to polish them. Gonna replace them with a new set anyway I think along with the wires, cap and fresh plugs. This mag has a prestolite tag on the side instead of a wico tag so I would guess its a late replacement part. The carb has no model number cast in it, only a tag on top thats stamped 108u. From what I can tell this is also a late replacement carb after Marvel Schebler condensed there two cylinder line down to three universal models. Tractor was stored for 20 some years before I got it so I would wager both were installed shortly before then since everything on both shows so little wear.

John Deere A magneto

MikeinInd

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D beatty

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John B.

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John T

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Ken L

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red.green

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Earl-IL

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MikeinInd

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