H4 Magneto question

JonCT · Apr 17, 2021

I was checking the timing on my 39 M today and when I rotated the mag toward me to trip at the timing mark it zapped the living heck out of me! I think it was through the screws that hold the coil cover on...not sure though. What would cause that?

teddy52food · Apr 17, 2021

Cracked cover?

SadFarmall · Apr 18, 2021

Does it have the early cable assembly to ground the magneto or a screw for a wire back to a grounding switch? If your hand is touching the screw when the magneto trips and the magneto is not grounded, it bites - you already know how I found that out
SadFarmall

pete 23 · Apr 18, 2021

Should have switch in off position no matter what style shut off
it has. If all was hooked up it could have started engine. If
coil wire or plug wires were unhooked, the magneto was seeking a
place to ground and it found you.

Easy way to ruin a coil if it snaps on a open circuit. The high
voltage has to go some where.

JonCT · Apr 18, 2021

(quoted from post at 02:29:26 04/18/21) Cracked cover?

Yes...found a crack in the cover.

So I checked the points, set the timing, adjusted the valves and cleaned the carburetor. The tractor starts right up but has a sputter to it that cleans up under load.
[video play=false:dd86a32103]https://forums.yesterdaystractors.com/videos/mvvideo73795.mov[/video:dd86a32103]

JonCT · Apr 18, 2021

Im thinking its carburation...but any help pointing me in the right direction would be appreciated.

SadFarmall · Apr 19, 2021

Very true it could start, but those sort of things don't cross your mind at twelve or thirteen when you're trying to make something work (that you probably should not have touched in the first place) - and not be seen sneaking off with Dad's spanners and screwdrivers!
SadFarmall

JonCT · Apr 19, 2021

Good thing it went through my hand! Saved the coil!

SadFarmall · Apr 19, 2021

Forgive my ignorance if I'm on the wrong track, but isn't the safety gap meant to protect the coil on magnetos such as F4 and F6? I have had a rapid read through the section on H4 magnetos and I have not seen anything about grounding the magneto to prevent damage to the coil, though I may well have missed it.
SadFarmall

pete 23 · Apr 19, 2021

No safety gap in that magneto. One time on open circuit will
normally not ruin the coil, but it sets it up to happen. Lot of
lawn mower coils take a beating when you forget to hook the plug
wire back up after doing some repairs and then try starting it.

My former employer would remove coil to cap wire on tractors on
the lot so people could not start them. Iot of them got tried so
you know that was not good for the ignition system. Carbon tracks
around coil output terminal etc.

SadFarmall · Apr 19, 2021

That makes sense. Thank you!
SadFarmall

JonCT · May 27, 2021

Update...I found an intake leak, broken bolts & stripped threads along with cracks in the manifold.

Bought a new manifold and installed it..running much better now. I just need to put in the new carb kit I bought.

Brown Dirt Cowboy · May 27, 2021

not that it really matters but what is the serial# on your '39M? mine is# 1094. Thanks Tom

H4 Magneto question

JonCT

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pete 23

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JonCT

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