cub j4 magneto

[WayneMo]

I just bought a new coil for my cub j4 magneto. I have not installed it yet but question if the wire from the coil should show continuity to the high voltage nub on the coil. I measured 9 ohms resistance from coil wire to high volt nub. Should the coil wire be grounded when put in the circuit?
The [problem is that everything from the coil wire to the points shows grounded when connected and opening or closing the points makes no difference. Does the coil wire become hot when the mag is turning? Everything looks to have correct insulation inside the circuit. AmI missing something here? In a normal external coil system, the wire to the points is hot and grounds when the points close.

Another question I have is how often does the coil itself fire twice for every crank shaft rotation?

 

[wore out]

Your observations about Ohms and continuity between the high voltage terminal, the points and condenser terminal and ground are correct except for the 9 Ohm reading, you need to take a look at what scale the meter is set to, 9000 Ohms would make more sense. The coil ''fires'' twice per crankshaft revolution, when either of the two magneto cam lobes opens the breaker points. (The main shaft in the mag turns at crankshaft speed.)

Standard four-cycle engines fire ALL their cylinders over two crankshaft revolutions, so this 4-cylinder engine fires TWO cylinders each crankshaft revolution.

This post was edited by wore out on 12/16/2021 at 01:33 pm.

 

[WayneMo]

If I understand correctly, My new coil is ok and yes, the ohms are 9000 not 9. So right now, I have the coil mounted with wire connected to the points and condenser and also the kill terminal and everything shows grounded when the mag is not moving regardless of points opening or closing.
I will go ahead and finish putting the mag together and test fire it with the points adjusted to .013 inches. I just got flustered when everything appeared grounded regardless of point opening when stationary.
Thanks for your help. I bet few on this site understood this! I guess the coil voltage and current can be determined knowing the coil resistance.

 

[Janicholson]

Volt todays general purpose ohm meters don't measure decimal fractions of an ohm. So what appears to be a ground is actually being measured through the winding to ground. Jim

 

[wore out]

(quoted from post at 18:46:03 12/15/21) If I understand correctly, My new coil is ok and yes, the ohms are 9000 not 9. So right now, I have the coil mounted with wire connected to the points and condenser and also the kill terminal and everything shows grounded when the mag is not moving regardless of points opening or closing.
I will go ahead and finish putting the mag together and test fire it with the points adjusted to .013 inches. I just got flustered when everything appeared grounded regardless of point opening when stationary.
Thanks for your help. I bet few on this site understood this! I guess the coil voltage and current can be determined knowing the coil resistance.

Here's a quick (and crude) schematic of your magneto.

The "low" end of the coil primeay (A) is grounded to the magneto "frame".

When checking Ohms between "A" and "B" (the junction of the primary and secondary windings) you will read coil primary resistance with the breaker points "open" and (ideally) zero Ohms with the breaker points "closed".

Checking Ohms between points "B" and "C" will give you the secondary resistance of the coil.

Checking Ohms between points "A" and "C" (with the breaker points "open") will give you the series resitance of the coil's primary and seconday.

But, as you have already figured out, even with the breaker points "open" there's a certain amount of conductivity between "B" or "C" and the magneto "frame"/chassis ground.

 

[WayneMo]

I cannot believe the trouble I am having setting the points. I have done this on other distributors all my life and I am 84 now but I should not have so much trouble. Maybe I have the stationary contact misassembled with the parts that hold it.
I have the stationary point fit into the groove and the points line up. Then I have the small piece on with the bent up portion and then a washer with the screw holding it all together. How should the small piece with its bend be positioned? I don't see any way it can fit and make it easy to adjust the mag points!

 

[WayneMo]

Yes, you are no artist but I think I understand it. You are a great help!
I learned here that the mag only produces voltage to the coil, points, and condenser when
rotating. That seems obvious when you think about it!
My problem now is adjusting the points. They slide in a machined groove to adjust and there
is nothing nearby to assist a screw driver in moving them back and forth like in other
distributors I have worked on.

 

[WayneMo]

The mag fires twice per revolution so it must fire on both the power and exhaust stroke. I think it rotates clockwise looking into the engine drive. Hope mine works when I finally get it installed!

 

[teddy52food]

Set the points wide with the screw just loose enough so it will slide. Close the gap with a pliars.