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What am I missing on this magneto?

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clodhopper
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What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 1:26 pm

This is a summary of my situation. I have a 1950 cub that runs like a top. It had a magneto with an external coil. I recently purchased and rebuilt a magneto, as I wanted to bring it back to original. The magneto was disassembled completely, cleaned and reassembled. Impulse coupling is good,new brillman coil that has been bench tested, new points (filed to remove hard surface)and gapped at .013, known good condensor, new pinion gear and rotor button, known good cap. Please keep in mind this tractor was running perfectly, so fuel is not an issue, plug wires and plugs not an issue. With the magneto installed, i have very good spark from the coil. When testing spark between wire terminal and spark plug, I have fire on cylinder 1 and 4, and no fire on cylinder 2 and three. How is this possible? Thanks for the help in advance!
EDIT- The magneto is timed correctly, and timed to the engine correctly

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John *.?-!.* cub owner
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Re: What am I missing on this magneto?

Postby John *.?-!.* cub owner » Sun Jul 19, 2015 1:41 pm

My first thoughts are the impulse coupling. There are 2 dogs 180 degrees from each other that should trip it every half round of the magneto. If one of them is not working, that would give you the indications you described. Note, while there arte springs to push the dogs out, they are not very strong, and gravity assists them. After that I would check the points gap on all 4 lobes of the shaft for a worn lobe or worn shaft bushing. I am assuming you are testing for spark using a spark plug or similar device, and not some type of electronic tester, since a magneto fires with opposite polarity every other time, and will fool some electronic devices.
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clodhopper
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Re: What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 1:57 pm

The impulse coupling does trip every half rotation. I am using a spark tester grounded to the top of the spark plug. I have not checked the point gap at all four lobes of the shaft. Is a bent shaft a possibility?

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Re: What am I missing on this magneto?

Postby Cecil » Sun Jul 19, 2015 2:44 pm

Test the mag from the coil cap. This lets you know that is continuously firing. There should be a spark every time the impulse trips. If good there the problem is with the wires or cap.

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clodhopper
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Re: What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 3:31 pm

I have put it back on the bench. Testing it straight from the coil I have verified that about 60% of the time it is firing only once per revolution. Swapped out the points and still get the same results. Listening to the arc of the spark, you can hear the difference in the cadence when it sparks correctly, or skips a spark. My only estimation can be a shaft problem. Visibly, there is nothing wrong, but the spark doesn't lie, and everything else has been taken out of the equation.
Edit- speaking to John's earlier post, you can hear each of the dogs clicking every revolution.
Last edited by clodhopper on Sun Jul 19, 2015 3:40 pm, edited 2 times in total.

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clodhopper
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Re: What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 3:37 pm

I do have one more question. How in the world do you bend a magneto shaft?

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Re: What am I missing on this magneto?

Postby Jim Becker » Sun Jul 19, 2015 4:10 pm

Check the ignition spark gap 4 times, once on each lobe of the cam. If the shaft is bent (or loose) you will get a different gap on different lobes. If they all measure the same, this isn't your problem.

You haven't answered John's question about your spark tester. Is it just a mechanical gap or is it electronic and possibly giving misleading results?

And the impulse is locking in and giving a solid snap TWICE every revolution?

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clodhopper
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Re: What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 5:13 pm

Yes, I am using a simple spark gap tester, nothing electronic. The impulse snaps twice per evolution. On the bench, I have the spark tester grounded to the case, and am spinning the shaft backward with a motor, withe the points cover removed. The only thing operating is the coil, condenser, and points. If it is not the shaft, what else can it be?
Edit- the impulse coupling can be heard when installed on the tractor and cranked by hand.

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Re: What am I missing on this magneto?

Postby JoeB » Sun Jul 19, 2015 5:19 pm

J4 magneto is only going to have 2 lobes on the rotor shaft. I would try another cap, preferabbly a known working one. Check that the wires are snug in cap.

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Glen
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Re: What am I missing on this magneto?

Postby Glen » Sun Jul 19, 2015 6:12 pm

Hi,
I don't know if you have read this post below, the man wrote on his last post that he replaced the coil and coil cover and the magneto worked. Your new coil might be defective, so could the condenser. Hard to tell without being there what is wrong. Hope you get it to work. :)

viewtopic.php?f=1&t=89344

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Re: What am I missing on this magneto?

Postby Jim Becker » Sun Jul 19, 2015 6:13 pm

clodhopper wrote:. . . spinning the shaft backward . . .

????

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clodhopper
10+ Years
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Posts: 807
Joined: Sat May 07, 2011 8:20 pm
Zip Code: 27526
Tractors Owned: 1950 Cub Demo
1978 Cub Cadet 1650
cub 193 plow
cub 174 planter
cub sidedresser
cub 144 cultivator
cadet 50C deck
Circle of Safety: Y

Re: What am I missing on this magneto?

Postby clodhopper » Sun Jul 19, 2015 6:27 pm

You can spin the shaft backward, the coil and points still operate, and you don't have to overcome the impulse coupling.

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Re: What am I missing on this magneto?

Postby Jim Becker » Sun Jul 19, 2015 6:37 pm

But the magnet is in the wrong orientation when the points break.

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John *.?-!.* cub owner
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Re: What am I missing on this magneto?

Postby John *.?-!.* cub owner » Sun Jul 19, 2015 8:25 pm

Jim Becker wrote:But the magnet is in the wrong orientation when the points break.
Agreed, I do not think you will have a dependable spark in reverse.
If you are not part of the solution,
you are part of the problem!!!

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Re: What am I missing on this magneto?

Postby Gary Dotson » Mon Jul 20, 2015 6:01 am

I agree with Jim & John! When spinning the mag in reverse, the points break at the wrong time. I'm surprised you're seeing any spark at all.


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