My Magneto advance doesn't work

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Paul Van Benschoten
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My Magneto advance doesn't work

Postby Paul Van Benschoten » Tue Jun 10, 2014 7:12 pm

I have been following the discussions in several threads about setting up the governor and the governor to carb connection. This helped me get a new-to-me 1950 cub running smoothly.

I have a magneto that I static timed. Now I can start the tractor with the hand crank ! :{_}: I did attach a timing light just because I have one and I need to justify keeping it around. :D As I moved the throttle, the notch on the pulley did not move at all.

I looked at the Engine Electrical manual and the Magneto manual but I did not see anything about the advance weights. The magneto manual on this site doesn't list the J-4, but I thought they may have something about the weights in general.

Is the magneto "easy" to disassemble to get to the weights? I did open the mag to replace the points and condenser as well as the rotor. I assume that the advance is numbers 50, 51, 52, 54 and 55 in this picture. If I remove the end plate number 53, will tiny parts come flying out? Or do I get at the weights by removing numbers 30 - 37 ?

IH mag.JPG
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Paul Van Benschoten

Eugene
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Re: My Magneto advance doesn't work

Postby Eugene » Tue Jun 10, 2014 7:33 pm

And NO, there is no advance in a magneto that you would see in a standard battery powered ignition system.

The advance is actually a retarding mechanism called the impulse coupling. The impulse coupling sets the spark at zero degrees, TDC, when cranking over the engine. Once the engine starts and achieves around 250 rpms, the impulse coupling drops out and the magneto immediately goes to "full advance".
I have an excuse. CRS.

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Rick Spivey
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Re: My Magneto advance doesn't work

Postby Rick Spivey » Tue Jun 10, 2014 8:08 pm

Eugene has you covered. As best I can tell, the magneto has a built-in advance of approximately 13 degrees, maybe even the same 16 degrees as the battery ignition. However, if you tried to hand crank with the timing advanced like that, there could be a kickback that is known to break wrists or thumbs. So the impulse coupling (the parts you guessed as weights) actually retards the timing for hand cranking. Once running, the impulse drops out due to centrifugal force, and the magneto is at full advance. Do NOT attempt to break into the impulse coupling without the impulse puller tool, and a bit more knowledge about what you will find. I learned them the hard and expensive way, when I ruined one and had to find replacement parts. The parts are getting much harder to find.

So static time your mag as shown on this forum, and then leave it be.
Rick Spivey
'52 Cub ("Great Personality") 148xxx
'48 Cub with FH ("Gunny Cub") 38xxx
'57 Lambretta (a slow work in progress)
'74 Triumph TR6 (Mama's toy)

Paul Van Benschoten
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Re: My Magneto advance doesn't work

Postby Paul Van Benschoten » Tue Jun 10, 2014 8:54 pm

Great! I'll leave it alone and move on to another project.

Thanks for the quick responses.
Paul Van Benschoten


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