J4 Magneto

Well I thought my day was going pretty good, getting my latest acquisition ready to start. Temp fuel...Check. New battery wires and starter switch....Check. Put the exhaust pipe back on... Check.. Crank her up.........Magneto No Check..#@@@*^()**+. I've been playing with this M for about a month now,taking my time and I'm just now noticing that this tractor has a J4 mag on it instead of a H4..(scratching head) OK, no big deal. they are sort of interchangable. But when I got out the multimeter to troubleshoot,, I discovered that with nothing hooked up to the grounding post I was still getting continuity between that and the chassis. Upon opening it up I did not find anything that looked like it was grounding out at all.. So I'm stumped. I am correct in assuming that I should not be getting a ground at the post outside the case, Right??? The coil reads solid, the points are new, and the condenser is insulated from the case. What am I missing here? :?

If the points are closed there will be continuity to ground through the stationary point. Make sure the points are open and try again. Also the coil is grounded to the mag case through the strap under the coil mounting screw.
Chuck

Even WITH the points "open" there is a relatively low resistance path from the "kill" terminal to ground through the coil's primary winding.

Forgot about that "path", did you?

Still just could be the condenser. Incidently I ran an H with a J4 mag for years. MTF

If its a mag grounding kill switch they are connected to the points.

THEREFORE when points are CLOSED Id expect:

Near 0 ohms (short) to case/frame/ground

when points are OPEN Id excpect

Very Low Ohms (0.25 to 1.5 ohms) to case/frame as thats the low voltage primary winding of the coil. (If open the coil is badddddddd)

AS BOB CORRECTLY NOTED BELOW

Id expect maybe 3000 to 10,000 ohms from the HV output to case/frame as thats the coils HV secondary winding. If its an open circuit its badddddddd


NOTE it takes a good ohm meter to read those real low ohms accurately as a cheap one may look like its zero ohms points open or closed. Use the R x 1 scale

MAKE SURE the grounding stud isnt shorted to ground nor the wire from it to the points

NOTE theres probably a condensor in paralell with the poonts connected to case/frame/ground. If its shorted out that kills the spark (and the ohms to case/frame will ALWAYS read zero points open or closed). If its totally bad/open she can still fire but a weak spark.

Stick a GOOD ohm meter on the low ohms R x 1 scale on the kill terminal and insure its NOT always a dead short and see if its near 0 ohms when points are CLOSED but 0.25 to 1.5 or so ohms when OPEN..........

John T

forget the volt meter. Clean the points. check lead for good blue spark should jump at least a quarter inch. could be condenser, could be coil but always check points first. They are #1 culprit for poor spark.

Thanks guys, thats what I was hoping... I've got no spark at all right now so I was going down the list of possibilities. Thats when I noticed that the grounding post was hot with the chassis with no switch hooked up. I didnt try opening the points first tho...I don't know about the condenser yet either,But I'll change it out anyway..I do know the coil is good tho, at least the expensive part still works.. i"m also glad the J4 is interchangable..

The engine will run with a J-4 but the timing advance is much different than an H-4.

Good Tip, I'll check on that after I figure out where my spark went. I went out and found me one of those icepick Spark testers at napa. I'll try it out tommorrow.Thanks...

Peace

J4 mag timing is adjustable 0 to 30 degrees. It will trip in exactly the same place that will give you TDC and providing its set to -30 degrees BTDC its the same. MTF

Mike, how do you adjust the timing from 0 to 30 degrees on a J4? I thought it was fixed by the impulse plate.

I was going to ask that same question.. All my manuals say you set everything the same, way thru the impulse plate.. :?

Its done by moving the position of the index plate. Remove the impulse coupling, this will allow you to see the index plate. The plate is held in place with two screws. The plate and the magneto body should be marked. Remove the screws and turn the plate in the required direction to line up with the amount of advance that your tractor requires. Replace the screws and the coupling. Job Done. MTF

Mike, are you talking about International J4 magnetos? I have had several apart and they did not have an index plate as you described.

OK guys. Hand on heart for this one, my mistake. No I'm talking about the R4. Call it an age related thing. I would appreciate it if you leave the shotguns hanging up, cos I'm obviously a bit feeble. Sorry again Mike

(quoted from post at 11:49:09 02/15/10) ........ No I'm talking about the R4. Call it an age related thing.......

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ike, I understand that all to well, believe me I do.

Thanks for the clarity.I thought I was Blind and senile.No stone throwing from here my friend...

(quoted from post at 14:48:29 02/10/10) Good Tip, I'll check on that after I figure out where my spark went. I went out and found me one of those icepick Spark testers at napa. I'll try it out tommorrow.Thanks...

Peace

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oax, be sure to check that tester on a known good system, I bought one about a month ago and found the low voltage side was ok, but ignition side did not work.