J4 Distributor Rotor - Broken teeth

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NZ_Cub
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J4 Distributor Rotor - Broken teeth

Postby NZ_Cub » Sat Jan 24, 2015 11:45 pm

Just replaced our Cubs J4 distributor rotor due to the center tab being broken/worn off. Fitted new rotor, lined up the marks etc put the original distributor cap on and tired to start it, fired on one cylinder then nothing, thought it was strange.......

After pulling off the cap etc and taking out the rotor it appears to have hit a terminal in the cap and the small cast gear had smashed off 2 teeth on the rotor.

Is this cased by the worn cap points? as the new rotor contact was a lot longer than the old worn one?

Stuck on a solution, don't want to smash another rotor. do we need to go for a new cap to?

Cheers.

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Re: J4 Distributor Rotor - Broken teeth

Postby tst » Sun Jan 25, 2015 9:03 am

were the teeth greased where they mesh?, crank it over with out the rotor on it to make sure the mag gear is spinning freely with out binding, perhaps you have the wrong rotor?

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Re: J4 Distributor Rotor - Broken teeth

Postby Rick Spivey » Sun Jan 25, 2015 4:57 pm

If J4 magneto, rotor needs grease where it rides on stud, and needs grease where it meshes with pinion. Pinion usually wears out long before rotor, so make sure pinion teeth are full, not sharp. Lastly, time rotor to pinion using match marks, replace rotor/pinion cover. Should NOT be an issue with cap, much more suspect of no/poor lubrication, worn pinion.
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John *.?-!.* cub owner
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Re: J4 Distributor Rotor - Broken teeth

Postby John *.?-!.* cub owner » Sun Jan 25, 2015 8:06 pm

Use a caliper or similar tool to compare the length of the arm from the center of the rotor. Since practically all of the parts, especially for the old stuff is imported, it is not unusual for a new part not to be correct.
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Re: J4 Distributor Rotor - Broken teeth

Postby NZ_Cub » Mon Jan 26, 2015 12:43 am

Suspect worn pinion as the teeth do look sharp. will replace this anyway.

Grease was applied to the rotor and on stud as it felt tight to turn by hand.

The old rotor turned freely when the engine cranked over. this is when we noted the center tab was broken off the original rotor.

So the old cap will be fine? even with the new rotor contact being allot longer and contact further out on lead points?

Cheers for the help.

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Re: J4 Distributor Rotor - Broken teeth

Postby Harold R » Mon Jan 26, 2015 8:42 am

Is this what your new rotor looks like? This is the one for a J-4 Cub magneto.

Image
Photo courtesy of TM Tractor.

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Re: J4 Distributor Rotor - Broken teeth

Postby tmays » Mon Jan 26, 2015 8:48 am

Harold R wrote:Is this what your new rotor looks like? This is the one for a J-4 Cub magneto.

Image
Photo courtesy of TM Tractor.

Yep, and they are right proud of those little boogers :-)
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Re: J4 Distributor Rotor - Broken teeth

Postby NZ_Cub » Mon Jan 26, 2015 1:59 pm

Yep, rotor correct and as per picture. The box it was in said 'J4 ROTOR' on it.

I suspect the cap is not the true Cub magneto cap? is there a way of checking any measurements to see?

The Pinion is worn and sharp. We will replace this also.

But the rotor seems to run allot further outwards on the cap points that the old one, as if the existing rotor had been trimmed? or is this normal wear?

Thinking back to taking the broken rotor of the first time, there was only one screw in the cover that holds the rotor in position. would this have caused uneven wear on the cap points?

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Re: J4 Distributor Rotor - Broken teeth

Postby Harold R » Mon Jan 26, 2015 6:17 pm

I just can't quite picture your predicament, however, I have seen some repair shop drill out the old contacts on a distributor and insert copper studs. Those may not have been fitted well and rubbed the rotor slightly......just a guess. I have a working mag, here are some pics to compare.
Image
Image
Image

NZ_Cub
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Re: J4 Distributor Rotor - Broken teeth

Postby NZ_Cub » Tue Jan 27, 2015 12:48 am

All looks the same as the cap etc we have, can't get pics to compare as it's at my parents place 45 mins away.

Can you please measure the length of the copper tap the contacts the lead points? the one we have seems longer from memory.

Harold R wrote:I just can't quite picture your predicament, however, I have seen some repair shop drill out the old contacts on a distributor and insert copper studs. Those may not have been fitted well and rubbed the rotor slightly......just a guess. I have a working mag, here are some pics to compare.
Image
Image
Image

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John *.?-!.* cub owner
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Re: J4 Distributor Rotor - Broken teeth

Postby John *.?-!.* cub owner » Tue Jan 27, 2015 9:34 am

I did not mention this sooner, since it was a slightly different circumstance, but will now. When I was rebuilding my very first cub (25+ years ago), which was a non runner when I got it, it needed a new cap and rotor. Back then the rotor was approximately $20, and an IH cap was $6. I could get a cap from O-reilly for $2.50, so I did. First time I fired it up I broke the rotor just like you did. Another new rotor and an IH cap and all was well. With your cap being existing rather than a new aftermarket I did not mention it sooner.

Before you fire it up with a new cap and rotor, remove the magneto. That way you can turn it backwards without winding the impulse to make sure it will not strike the contacts and bind. You can also verify it turning forward by using a small screwdriver or knife blade and holding the impulse trip pin back as it passes the stop in the housing each time (twice per round).
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Re: J4 Distributor Rotor - Broken teeth

Postby Eugene » Tue Jan 27, 2015 1:56 pm

John *.?-!.* cub owner wrote:Before you fire it up with a new cap and rotor, remove the magneto. That way you can turn it backwards to make sure it will not strike the contacts and bind.
My suggestion also.
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Re: J4 Distributor Rotor - Broken teeth

Postby challenger » Tue Jan 27, 2015 2:40 pm

It looks like the positioning tang along the outer ridge of the rotor cap is broken off. Does the cap fit real tight in the base and is it possible the cap was canted some when installed caused the rotor to hit an inside terminal? You should have a cap with a good tang on it that fits in the slot on the base because this is critical for proper timing.

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Re: J4 Distributor Rotor - Broken teeth

Postby Harold R » Tue Jan 27, 2015 5:48 pm

It looks like the positioning tang along the outer ridge of the rotor cap is broken off.

It is.
Does the cap fit real tight in the base

It does. It runs fine when I use it. It was NZ_CUB who had the foul fitting cap and rotor. I was just offering him some mode of comparison.
Here's the length of the brass, (or copper), contacts in the distributor. They look fairly standard to me. I understand you're quite a distance from your cub.....but a picture would surely help.
Image


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