Cub J-4 magneto

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ecbrewster
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Cub J-4 magneto

Postby ecbrewster » Sun Dec 07, 2014 4:56 pm

As a follow up from my previous post, I disconnected the points, capacitor, and coil wires from the kill switch post and verified that the post is insulated. Every thing looks good until the coil wire is connected, it then shows a direct path to ground on the body. In checking the coil that I removed to install the new one there is a direct path (or very low resistance, too low to measure) from the ground lug at the core of the coil to the lead at the points. Both coils measure the same. The new has been run less than 20 minutes prior to the problem that I had with no spark. What am I missing?

Thanks again!

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John *.?-!.* cub owner
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Re: Cub J-4 magneto

Postby John *.?-!.* cub owner » Sun Dec 07, 2014 8:10 pm

Both the wire that goes to the points and the wire going to the plugs is connected to the ground tab via very low resistance windings in the coil. Here is a diagram of a magneto and coil.

A note, the 2 small circles directly above the word coil are separate, not connected together as they appear in the diagram.

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Re: Cub J-4 magneto

Postby Bigdog » Sun Dec 07, 2014 10:40 pm

Magneto coils have very low direct current resistance so taking readings with an ohmmeter don't tell you a whole lot. I seem to recall the resistance of the coil is in the area of 1 to 2 ohms.
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ecbrewster
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Re: Cub J-4 magneto

Postby ecbrewster » Mon Dec 08, 2014 10:41 am

Thanks John and Big Dog, that was the information that I had been searching for. I'll keep my fingers crossed that when i put it back together it works.

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Re: Cub J-4 magneto

Postby Smokeycub » Mon Dec 08, 2014 10:50 am

I'm going to state the obvious here. There are two windings in the coil as is shown in Johns' schematic, and because one coil is new doesn't mean it isn't or hasn't gone bad. ...and because their is such low resistance it's hard to tell if one or the other circuit isn't bleeding off. I'm going to guess there's a leak (not an open circuit) in the secondary circuit greatly reducing or killing the spark, provided you've exhausted all other possibilities. In spite of what I just said, I can't help but feel your problem is in the primary wires between the points and the coil, an intermittent break or grounding.
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ecbrewster
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Re: Cub J-4 magneto

Postby ecbrewster » Thu Dec 11, 2014 10:05 am

Thanks for all of your inputs. Smokeycub hit it on the head. Although the new coil had been installed a short time ago, it must have a problem. I re-installed the old coil as a last ditch effort (after spending time switching out capacitors and points) and I had a beautiful blue spark. I don't know why I had changed the coil in the first place, I seemed to be chasing my tail. All is well that ends well,thanks again to all three of you!

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John *.?-!.* cub owner
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Re: Cub J-4 magneto

Postby John *.?-!.* cub owner » Thu Dec 11, 2014 11:33 am

If it is any consolation, I got hold of a couple coils that only lasted a short time 3 or 4 years ago. All of mine have been doing great ever since. As I assume you have learned over time, magnetos are more sensitive to moisture than distributors, and I have found that if mine have to set out side, or even under a lean to shed that wrapping a piece of plastic around the magneto and wires where they come out of the cap seems to help a lot. I even wrap them when going to shows where they will set outside in the dew. Since I started doing that, no more coil problems. My preferred wrap is the small bubble wrap.

As to your old coil, you may find that when the weather gets hot and you work it, you may have a starting problem when hot. That is not unusual when they age and may have been the reason it was replaced originally.
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Re: Cub J-4 magneto

Postby havoc1482 » Thu Dec 11, 2014 12:57 pm

To add to Johns post, don't forget to replace the gaskets around those things. My Super As mag shot itself when I left it in the rain one day. Water got into the points via the coil cover and fouled them up. I replaced the gasket on the cap and the one behind it where it mounts to the engine. I haven't had a moisture issue since and I've left it outside plenty of times.
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Re: Cub J-4 magneto

Postby Clark Thompson » Thu Dec 11, 2014 3:03 pm

I have had a lot of problems with after market magneto coils that I purchased 2 years ago. I found that if there is continuity from the wire from the coil to the points to the ground on the coil that there is a short between the primary winding and secondary winding.. Many times this will cause ignition failure with in a few hours.. I have had so much proplems with aftermarket mag coils in the past 2 years that I have been converting the mags to run off of a external coil or installing a battery ignition. 3 out of 4 aftermarket coils I have purchased have been defective.
Collector of Farmall cubs and cub cadets.Injoy helping people keep their cubs running. Years of experipnce.

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Re: Cub J-4 magneto

Postby Smokeycub » Fri Dec 12, 2014 5:22 pm

Clark Thompson wrote:I found that if there is continuity from the wire from the coil to the points to the ground on the coil that there is a short between the primary winding and secondary winding..

Thanks Clark - That's what I meant by "a leak"
Smokeycub wrote:I'm going to guess there's a leak (not an open circuit)
That's good information!

ecbrewster - Glad you got it going! :coffee:
Ray
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Attachments - 193 plow - 144 cultivator - 22 mower - 28A disc harrow - 54 leveling blade - Woods 59C2 - drag harrows - Mott D9 flail - flat belt pulley

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ecbrewster
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Joined: Wed Oct 24, 2012 10:44 am
Zip Code: 02631
Tractors Owned: 1948 Farmall Cub
Location: Brewster, MA

Re: Cub J-4 magneto

Postby ecbrewster » Fri Dec 12, 2014 7:58 pm

It was great to hear all of your inputs and ideas. You guys are a wealth of information. It shows that your never too old to learn! Thanks again.


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