Cub J4 Magneto Rotor Position

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katzandmice
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Cub J4 Magneto Rotor Position

Postby katzandmice » Fri Dec 15, 2023 7:15 pm

I have a question for those folks who are a bit smarter than I am regarding the J4 magneto on a Cub. First, keep in mind that this Cub ran. It is only being worked on now due to the fuel sitting in the tank too long clogged up all the fuel arteries. The magneto has not been removed, nothing. But it seems that the rotor is in the wrong position. You should see that the #1 cylinder is clearly at TDC (pic of timing mark and pic of the top of piston). However, and maybe it’s just me, but wow, that rotor looks like it is not in the right spot! From many videos I’ve seen online, I think the rotor should actually be pointing towards about 1 or 2 o’clock. Anyways, I’m just curious. Again, maybe it’s just me.
Attachments
Timing Mark at TDC.jpg
Timing mark
Mag Rotor Position - #1 Piston at TDC.jpg
Rotor position
#1 Piston at TDC.jpg
#1 Piston TDC

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tmays
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Re: Cub J4 Magneto Rotor Position

Postby tmays » Fri Dec 15, 2023 7:47 pm

Are you at tdc on compression stroke?
Thomas

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Re: Cub J4 Magneto Rotor Position

Postby Jim Becker » Fri Dec 15, 2023 8:45 pm

Somebody installed the magneto with the input shaft/rotor one half turn from where it should have been for proper timing. They then compensated for it by moving all the spark plug wires to positions in the cap one quarter turn from where they belong. Whether the wires should be a quarter turn clockwise or counterclockwise depends on the answer to the question Thomas asked. (Rotating the engine between TDC compression vs. exhaust would turn the rotor one half turn.)

Installing the magneto incorrectly and compensating by moving the plug wires has no effect on how the engine runs. It will run fine. The down side is that it is likely to cause confusion for the next person to work on it, in this case for you.

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Glen
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Re: Cub J4 Magneto Rotor Position

Postby Glen » Sat Dec 16, 2023 12:00 am

Hi,
If you want the magneto timed the original way, and the spark plug wires in the original positions on the distributor cap, start at the beginning, be sure the engine is turned to number 1 cylinder top dead center, or TDC.
Below is a page from the 1949 Cub owner's manual telling how to time the magneto to the engine.
Paragraph 2 tells how to find TDC for the number 1 cylinder. Do that first.
Then see where the magneto rotor is turned.
https://farmallcub.com/rudi_cub/www.cle ... age-32.jpg

The number 1 spark plug wire is originally in the top position on the distributor cap. Then the others go around the cap clockwise, looking forward at the cap from behind the cap.
The firing order is 1, 3, 4, 2.
Number 1 cylinder is the front cylinder, or where the front spark plug is on the engine.
The cylinders are numbered 1, 2, 3, 4, from front to rear.
The rotor is gear driven, it has a timing mark that aligns with the mark on the rotor drive gear.
You need to remove the plate the rotor is on, and then the cover holding the rotor on the plate, then you can see the rotor timing marks. That is a good time to clean the area under the cover, and put new grease on the gears.
Below are pictures.
Pic 1 is the rotor timing marks.
Pic 2 is the points and condenser under the plate.
The points may need filing or replacing, if old and burned from use. If they are burned, it can make the engine start harder than it should.
The points need the correct opening gap also. The point gap is .013"
Don't turn the engine with the plate over the points off, you can lose the magneto timing. Turn the engine only a small part of a turn, if needed, so the right point is open, to set the point gap.
Pages 30, 31, and 32, in the 1949 Cub owner's manual have magneto and point info. The manual is at the top of the page at Cub Info, then go to Rudi's Manuals.
Attachments
Cub magneto 4.jpg
Cub magneto 4.jpg (25.13 KiB) Viewed 2498 times
Cub magneto 8.jpg
Cub magneto 8.jpg (25.14 KiB) Viewed 2498 times

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katzandmice
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Re: Cub J4 Magneto Rotor Position

Postby katzandmice » Sat Dec 16, 2023 10:13 am

Thanks Thomas, Jim, and Glen. Quickly, the #1 cylinder is at TDC on compression - I did verify this. I was always of the mind that, as Jim said, that the engine will run fine in cases like this. I've always thought that as long as the #1 plug wire is aligned with the rotor (wherever it might be) as long as the #1 cylinder is at TDC and in compression and that the plug wires are then arranged in the proper order - all should be fine. Confusing as all hell in the future though. I'll run through what Glen offered and see what I find.

In the event that the rotor timing marks are off is it then necessary to remove the magneto and rotate the input/driver coupling CW to position the small timing mark (in pic just above) to the correct position, followed by retiming the magneto as described in the Cub Manual?

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tmays
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Re: Cub J4 Magneto Rotor Position

Postby tmays » Sat Dec 16, 2023 11:48 am

If it runs good, then no need to mess with it. Can just keep in mind or mark which tower is number one. Not a necessity as the number one plug wire is connected to what is now your number one tower. But if you’re like me, you’ll have to go in and get it ‘right’.
Last edited by tmays on Mon Dec 18, 2023 2:38 am, edited 1 time in total.
Thomas

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katzandmice
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Re: Cub J4 Magneto Rotor Position

Postby katzandmice » Sat Dec 16, 2023 6:36 pm

Well, I "went back in" and found the rotor timing marks misaligned by about 3 teeth. Fixed that. #1 is not right at the top. Will be interesting to fire it up after getting the fuel system all cleaned up. Thanks for the input.

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katzandmice
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Tractors Owned: 1948 Farmall Cub
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Re: Cub J4 Magneto Rotor Position

Postby katzandmice » Sun Dec 17, 2023 10:22 am

For those still interested, here is a pic of the rotor after making the adjustment to align the rotor's timing mark with the mark on the rotor drive gear. Yes, this is with #1 at TDC on compression.
Attachments
Rotor After Adjusting Gear Timing.jpg

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Re: Cub J4 Magneto Rotor Position

Postby Jim Becker » Sun Dec 17, 2023 12:07 pm

That looks like it might not be far off in that position. The plug wires need to go back to the standard positions (#1 at 1 o'clock). Have you checked the timing, as in where is the notch of the crank pulley when the magneto clicks?

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katzandmice
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Tractors Owned: 1948 Farmall Cub
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Re: Cub J4 Magneto Rotor Position

Postby katzandmice » Sun Dec 17, 2023 7:48 pm

To Jim, yes, all is good. Thanks for your insights.

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Re: Cub J4 Magneto Rotor Position

Postby Rick Spivey » Mon Dec 18, 2023 8:39 pm

katzandmice wrote:In the event that the rotor timing marks are off is it then necessary to remove the magneto and rotate the input/driver coupling CW to position the small timing mark (in pic just above) to the correct position, followed by retiming the magneto as described in the Cub Manual?


I suspect the problem will be hat the governor drive is off position and will need to be corrected. I find hat magnetos are not nearly as flexible about being timed in the wrong position as distributors are. You likely will need to align everything correctly in order to correct things.
Rick Spivey
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Glen
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Re: Cub J4 Magneto Rotor Position

Postby Glen » Tue Dec 19, 2023 2:25 am

If you need to do what Rick said above, here is some info about the gears.
Below are pics.
Pic 1 is how the magneto drive on the governor gear should be turned when the engine is at TDC.
They have said on here if it isn't turned to that position, the magneto can't be timed correctly.
You would need to remove the governor, and turn the gear so it will align correctly.
That's a good time to replace the oil seal, if it is leaking, or old.
Pic 2 shows the oil seal.
Pics 3 and 4 are from TM Tractor Parts.
Pic 3 shows the parts in the governor housing, the gear is on the right of the pic.
Pic 4 shows the timing mark on the governor gear, it is a stamped in dot on the left side of the gear.
It should align with marks on the rear facing side of the idler gear in the engine, that runs the governor, when the engine is turned to TDC.
The surface on the governor gear that the oil seal runs on should be flat and smooth. Sand it smooth with fine sandpaper if needed. Wipe off all sanding with a clean rag.
The seal lip should face forward. Be sure the lip doesn't turn backwards when putting the gear through the seal. Oil the seal lip and surface that it runs on before putting the gear through the seal.
TM Tractor Parts has a new seal. :)
Attachments
Cub gov gear.jpg
Cub gov seal.jpg
Cub gov seal.jpg (19.86 KiB) Viewed 2256 times
Cub gov gear 3.jpg
Cub gov gear 2.jpg


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