H4 Magneto Impulse Coupling Questions

DBond12 · Jun 5, 2019

Hello all,

I've finally finished off rebuilding the engine in my Farmall M. It's running again for the first time in about 15 years.

Now, my question is this: I've got 4" overbore stepped head pistons. The original pistons were 3 7/8" flat top and the tractor has the original kerosene head on it still. My understanding is that the impulse coupling should be giving around 35 degrees of advance, but that other impulse couplings were available for use with the high compression pistons. Is there anywhere that a fellow could still find these? The tractor runs good, but it just isn't as smooth as I would like it. I've got the valves set correctly and the timing is also correct, so I'm pretty certain that what I'm dealing with is the advance.

If anybody could help out locating these parts that would be fantastic.

Thanks very much!

gene bender · Jun 5, 2019

The mag fires on TDC then when engine runs advance goes to 35 and thats the way the H-4 works. Most of the
time you need to go to a dist so you can have advance before starting. I had that problem with a hi-dome
overbore piston. Engine would not start with the mag but put a dist in and away we go. There was a directivelong
time ago covering the problem when going to hi-dome pistons

pete 23 · Jun 5, 2019

Depending on how great the step head piston is will determine the spark advance you need. A 5000
altitude step head piston used with a kerosene head will give you right close to what a flat head
piston with a gasoline head. So, that would mean your 35 degree advance would be pretty close to
correct at high rpm's.

The whole problem is that at lower speeds that much advance is going to make for a rough running
engine. If it has a rain cap on the muffler it will be slapping back and forth. Those pistons
are getting slammed with ignition long before they like it at low speeds.

You could get a hold of a battery ignition unit in good shape and then you can play with the
timing to find where it runs smooth. That will tell you what advance is best at various speeds.
But, then you would need to tune that ignition unit for proper advance at the different speeds.
Gets complicated with all the different combinations of compression ratios that come about with
different heads and pistons.

The battery ignition distributors come in 40, 30, 25, and 22 for gasoline M's, H's , 300, 400's
etc.

DBond12 · Jun 6, 2019

Thank you both very much. Makes sense to me. Is there any way to give the mag less total advance? My understanding is that there is no advance curve with these units, but is there a way to get less total advance?

Not against going to a distributor, just that its expensive. And if I can get the old girl running smoother with the mag that I have I would prefer to do that.

teddy52food · Jun 6, 2019

If you can set it at WOT with a timing light, it would be good for a working engine. But it will be retarded for starting. Try it, it may fire up OK?

H4 Magneto Impulse Coupling Questions

DBond12

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gene bender

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pete 23

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DBond12

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teddy52food

Well-known Member

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