J4 Magneto Question

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cjack
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Tractors Owned: 49 Farmall Cub
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J4 Magneto Question

Postby cjack » Sun Feb 24, 2019 4:42 pm

We have a J4 magneto. The tractor has not run in 14 years and it is possible the magneto has been fired in the meantime without proper grounding. We followed all the instructions in other threads to set the timing, etc., but did not get any spark.

After investigation, it appeared that the grounding terminal was permanently grounded - there was very little to no resistance between ground and the terminal (0.1 Ohms to 0.0 Ohms). We opened up the magneto to look for shorts. Eventually we found that the coil wire was effectively grounding the terminal.

Just measuring resistance of the coil by itself, between the wire and the terminal on the coil that attaches to the bar we get 0.1 Ohms to 0.0 Ohms. Between the contact on top of the coil and the terminal on the coil that attaches to the bar we get about 9 k Ohms. From what I can find, the second reading is OK, but the first one means our coil is shot. I have read that the first reading should be about 2 to 5 Ohms (in which case the coil might be OK, but would not be OK with no resistance).

Is this correct?

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Rick Spivey
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Re: J4 Magneto Question

Postby Rick Spivey » Sun Feb 24, 2019 8:11 pm

I always check the coil independently of the housing. I check ohms between the wire that connects inside the mag body, and the nub that the cap touches. Then I check from the nub to the ground terminal that is fixed under the coil hold down screw. In both cases I am looking for a reading between 900 and 10500 ohms (pretty sure that is the scale I am on). Your problem may have been that the points can glaze over after sitting, or be dirty from handling, and you will get no spark. Just dealt with this issue at DSCf, and I was amazed that I had forgotten to wipe the points clean after installation.
Rick Spivey
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John *.?-!.* cub owner
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Re: J4 Magneto Question

Postby John *.?-!.* cub owner » Sun Feb 24, 2019 8:18 pm

14 years of non running just about guarantees the points are corroded unless it was stored under climate controlled conditions. Right behind that would be a bad coil.
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cjack
Posts: 37
Joined: Sun Sep 02, 2018 12:12 pm
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Tractors Owned: 49 Farmall Cub
Location: Burlington, ON, Canada

Re: J4 Magneto Question

Postby cjack » Sun Feb 24, 2019 9:40 pm

The points were a mess of corrosion. We cleaned them with some fine emery cloth and set the gap to 0.013". Still no spark.

I think I am going to order new coil, condenser (it looks rough too - insulation fraying), points and also spark plug wires.

The rotor and cap were replaced shortly before the tractor was parked - they are like new.

Rick, the resistance readings I get are
Wire to nub: 9130 Ohms
Nub to ground: 9130 Ohms
Wire to ground: 0.1 Ohms

One thing I forgot to mention - we are hand cranking while looking for the spark. One person hand cranks the other checks for spark. That should be OK, right? I figure there should be a spark regardless of whether you use the hand crank or the starter motor. Correct?

staninlowerAL
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Re: J4 Magneto Question

Postby staninlowerAL » Sun Feb 24, 2019 9:55 pm

Does the mag click when you turn the engine? After 14 years sitting the advance might need repairs. Look up details in GSS1012.

cjack
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Tractors Owned: 49 Farmall Cub
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Re: J4 Magneto Question

Postby cjack » Sun Feb 24, 2019 10:17 pm

Yes, click is fine. We were able to time the engine following Hengy's instructions in the how-to section. Aside from the points, the inside of the mag looked remarkably good.

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Glen
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Re: J4 Magneto Question

Postby Glen » Mon Feb 25, 2019 12:25 am

Hi,
You could grease the rotor gears, while the magneto is apart. The Cub owner's manual says to do it. Remove the cover held on with 2 screws, and clean the area, if the grease is old, and grease the gear teeth.
Also check the rotor and drive gear alignment marks. Below is a pic. :)
Attachments
Cub magneto 4.jpg
Cub magneto 4.jpg (25.13 KiB) Viewed 602 times

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Rick Spivey
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Re: J4 Magneto Question

Postby Rick Spivey » Mon Feb 25, 2019 7:13 am

cjack wrote:Rick, the resistance readings I get are
Wire to nub: 9130 Ohms
Nub to ground: 9130 Ohms
Wire to ground: 0.1 Ohms

One thing I forgot to mention - we are hand cranking while looking for the spark. One person hand cranks the other checks for spark. That should be OK, right? I figure there should be a spark regardless of whether you use the hand crank or the starter motor. Correct?


Those readings would indicate (to me) that the coil may very well be good. Those readings alone do not guarantee the coil is good, and I don't have any other coil testing capability other than try it in a magneto. The hand crank is sufficient to produce a spark, but I don't know what method you are using to see the spark, or at what point in the ignition system. All of the suggestions above are good ones to follow through on, as well as keep systematically eliminating possible causes. Check rotor/pinion alignment, especially if anyone has had that small bakelite cover off. Insure points and condenser are okay, double check the stud connections where the points/condensor/coil connect to be sure it is not grounded to the mag body. I would confirm spark directly at the coil wire first, if possible. Then chase distributor cap or wires if problems continue.
Rick Spivey
'52 Cub ("Great Personality") 148xxx
'48 Cub with FH ("Gunny Cub") 38xxx
'57 Lambretta (a slow work in progress)
'74 Triumph TR6 (Mama's toy)

cjack
Posts: 37
Joined: Sun Sep 02, 2018 12:12 pm
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Tractors Owned: 49 Farmall Cub
Location: Burlington, ON, Canada

Re: J4 Magneto Question

Postby cjack » Mon Feb 25, 2019 9:21 am

Thanks to all. We are testing spark by unplugging the middle wire from the distributor cap and holding it close (quarter of an inch) to bare metal on the engine block.

What is the procedure to confirm spark at the coil wire?

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John *.?-!.* cub owner
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Re: J4 Magneto Question

Postby John *.?-!.* cub owner » Mon Feb 25, 2019 2:38 pm

That is pretty much it.
If you are not part of the solution,
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Glen
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Re: J4 Magneto Question

Postby Glen » Mon Feb 25, 2019 6:50 pm

Hi,
You could put in new points and condenser first, and see if it works. They cost less money than a new coil, or put in a new coil too, whichever you want.
Not sure if you checked the insulation piece where the ground wire attaches on the side of the magneto, the people on here have said the insulation can crack or break, then it shorts and there is no spark.
TM Tractor has new ones, below is the listing, you can see pics of it.
Evidently there are some magnetos it doesn't fit, the listing shows the serial numbers it fits.
Check the serial number on the magneto, if you need to buy one. The number probably starts with J4. :)

http://www.tmtractor.com/new/el/957fp.htm

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mva1958
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Re: J4 Magneto Question

Postby mva1958 » Mon Feb 25, 2019 7:07 pm

cjack wrote:The points were a mess of corrosion. We cleaned them with some fine emery cloth and set the gap to 0.013". Still no spark.

I think I am going to order new coil, condenser (it looks rough too - insulation fraying), points and also spark plug wires.

The rotor and cap were replaced shortly before the tractor was parked - they are like new.

Rick, the resistance readings I get are
Wire to nub: 9130 Ohms
Nub to ground: 9130 Ohms
Wire to ground: 0.1 Ohms

One thing I forgot to mention - we are hand cranking while looking for the spark. One person hand cranks the other checks for spark. That should be OK, right? I figure there should be a spark regardless of whether you use the hand crank or the starter motor. Correct?


You might also check to make sure the little plastic insulator around the kill switch post is intact; I had one that was cracked and the kill switch couldn't be deactivated.

Also, and this isn't related to your spark problem, did you prime the oil pump?
Last edited by mva1958 on Mon Feb 25, 2019 7:28 pm, edited 1 time in total.
If only Mrs. Hoyt and Mrs. Clagwell had gotten along...

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Rick Spivey
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Re: J4 Magneto Question

Postby Rick Spivey » Mon Feb 25, 2019 7:26 pm

mva1958 wrote:
cjack wrote:The points were a mess of corrosion. We cleaned them with some fine emery cloth and set the gap to 0.013". Still no spark.

I think I am going to order new coil, condenser (it looks rough too - insulation fraying), points and also spark plug wires.

The rotor and cap were replaced shortly before the tractor was parked - they are like new.

Rick, the resistance readings I get are
Wire to nub: 9130 Ohms
Nub to ground: 9130 Ohms
Wire to ground: 0.1 Ohms

One thing I forgot to mention - we are hand cranking while looking for the spark. One person hand cranks the other checks for spark. That should be OK, right? I figure there should be a spark regardless of whether you use the hand crank or the starter motor. Correct?


Starter? If your tractor has the battery ignition option installed I think the points gap should be .20, mag notwithstanding.


Starter does not mean that battery ignition has been installed. He states specifically it is a magneto. Even if a magneto has been converted to external coil, like battery ignition, it is still gapped at 0.013". That keeps the proper timing function intact. If I misunderstood your post, please accept my apology.
Rick Spivey
'52 Cub ("Great Personality") 148xxx
'48 Cub with FH ("Gunny Cub") 38xxx
'57 Lambretta (a slow work in progress)
'74 Triumph TR6 (Mama's toy)

staninlowerAL
5+ Years
5+ Years
Posts: 3969
Joined: Fri Oct 29, 2010 11:34 pm
Zip Code: 36558
Tractors Owned: Cubs: (3)'49's, (1 is parts), (1)'57 IH Cub LoBoy w/FH, (2)154 Number Series Loboys, (1 is parts), '76 Longstripe w/FH, Mowers: C-22, Bush Hog 412, Pennington 59, Woods RM42CF, Woods 42, assorted FCub plows, planters, discs, etc. OTHERS: '49 AC B & Ind. Sickle mower, '61 AC D12 Ser 2, '52 8N, '56 Ferguson 35 Deluxe, '53 MM BG (offset), (2) '49 Avery V's, '51 JD M (regular), '56 JD 420C, with Blade and fire plow, '85 JD 850 (Yanmar) w/72" belly mower, '76? Yanmar 2TR15 1500, '78? FORD Dexta, '86 FORD LGT14D & 48" Mower, (4) Sears Surburban's, MTD's, Jeeps & other misc. "treasures"
Circle of Safety: Y
Location: AL (Southwest)

Re: J4 Magneto Question

Postby staninlowerAL » Mon Feb 25, 2019 7:30 pm

Mag points gap .013, battery/distributor points gap .020 Mag with external coil still .013 points gap.

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mva1958
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Re: J4 Magneto Question

Postby mva1958 » Mon Feb 25, 2019 7:32 pm

Rick Spivey wrote:
Starter does not mean that battery ignition has been installed. He states specifically it is a magneto. Even if a magneto has been converted to external coil, like battery ignition, it is still gapped at 0.013". That keeps the proper timing function intact. If I misunderstood your post, please accept my apology.


You read it correctly, Rick - I did a little more research and just edited out the bit about gap in my response.
If only Mrs. Hoyt and Mrs. Clagwell had gotten along...


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