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What Am I Missing? - C Timing Gear Train

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Pharmall

06-27-2005 06:01:14

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Sitting around waiting for a bearing puller to arrive, electrolytically cleaning parts, and just trying to beat the heat and drought here in southern New England. Started going through the H4 magneto on a '48 C and something isn't making sense.

Looking from the front of the tractor, the crankshaft turns CW, the camshaft turns CCW, the governor gear turns CW, which wants to turn the drive side of the magneto CW. But as I hold the mag in my hand, it seems that it wants to turn CCW. I get the pressure buildup, and then the snap when I turn it CCW. If I try to turn it CW it just feels like I am winding up a spring.

Obviously, I am missing something because this tractor was running prior to the rebuild. Most likely there is a short circuit between the ears that I haven't been able to find.... Jeff

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Bob

06-27-2005 07:56:49

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Re: What Am I Missing? - C Timing Gear Train in reply to Pharmall, 06-27-2005 06:01:14

The magneto has what is known as an "impulse coupling". That is the spring you feel winding up when you turn it CW.

When it has been turned CW far enough, the impulse will trip with a sharp "SNAP", and the mag will fire.

This "wind up and trip" action serves 2 purposes... it retards the timing for safe starting without "kickback", and it produces a HOT spark for starting.

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Pharmall

06-27-2005 08:20:51

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Re: What Am I Missing? - C Timing Gear Train in reply to Bob, 06-27-2005 07:56:49

Thanks Bob. When I was turning it CCW I was mistaking the magnetic drag for the "snap". Didn't realize that the spring tension was that high - felt like I was going to break something when trying to turn it CW, so I just wasn't turning it far enough. Jeff

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El Toro

06-27-2005 15:06:10

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Re: What Am I Missing? - C Timing Gear Train in reply to Pharmall, 06-27-2005 08:20:51

Good thing you were not holding the coil wire. Hal

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Pharmall

06-27-2005 20:28:56

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Re: What Am I Missing? - C Timing Gear Train in reply to El Toro, 06-27-2005 15:06:10

Had a senior moment on the rotation direction, but fortunately thought about the spark before cranking it over. That must be a nasty little jolt...

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El Toro

06-28-2005 03:14:09

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Re: What Am I Missing? - C Timing Gear Train in reply to Pharmall, 06-27-2005 20:28:56

Those mags will bite you pretty good if they're firing right. Hal

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