Mag issues

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jrsugg
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Mag issues

Postby jrsugg » Sat May 14, 2016 9:04 am

I have a 1948 cub with a j4 mag I am having trouble with the timing I have to adjust the mag to get it to start but then I have no power so I have to adjust it while its running to get power is there a advance in the mag that's not working , any help is appreciated thanks John

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Re: Mag issues

Postby Eugene » Sat May 14, 2016 9:20 am

Do you hear the magneto click when turning over the engine?

If not, impulse coupling problem.
I have an excuse. CRS.

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John *.?-!.* cub owner
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Re: Mag issues

Postby John *.?-!.* cub owner » Sat May 14, 2016 10:05 am

the impulse (snap Eugene referred to) delays the armature of the mag while a spring winds, then releases it to give the magnets a quick spin to start the engine. Once the magneto reaches approx 200 to 300 rpm the weights pull the arms back and the impulse no onger engages which also provides the spark advance. A magneto only has fully retarded or fully advanced, no gradual advance as a distributor does.

Have you by chance converted the tractor to 12 volts still using the original starter. It is rare, but I have heard of that set up spinning an engine fast enough the impulse would not engage.

Note, the snap of the impulse is hard to hear when cranking with the battery, to hear it you need to turn the engine slower such as with a hand crank.
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Re: Mag issues

Postby ScottyD'sdad » Sat May 14, 2016 10:13 am

I have a power unit on my cordwood saw, with a mag on it. No problem, starting it with 12 volts.
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Re: Mag issues

Postby jrsugg » Sat May 14, 2016 10:41 am

Yes the impulse is working I static timed it and everything is ok it will start better if I pull it and yes it has been converted to 12 volt

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Re: Mag issues

Postby Eugene » Sat May 14, 2016 11:16 am

jrsugg wrote:Yes the impulse is working I static timed it and everything is ok it will start better if I pull it and yes it has been converted to 12 volt
Still a magneto problem.

Timing light. Check the spark location while cranking over the engine. Should be at the TDC mark on the pulley. Check the spark location with the engine running.

Also roll the engine over by hand until the magneto snaps. Then check to see if the rotor is at the #1 spark plug tower location on the distributor cap. If not, one more full engine rotation. After magneto snap, roll the engine backwards until the pointer lines up with the TDC mark on the pulley. Now check the location of the rotor. Rotor should be pointing at, centered on the #1 plug wire location of the distributor cap.
I have an excuse. CRS.

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Re: Mag issues

Postby jrsugg » Sat May 14, 2016 12:06 pm

Yes the mag timing checks out when I static time it I will out a timing light on it thanks

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Re: Mag issues

Postby RaymondDurban » Sat May 14, 2016 3:00 pm

What is the compression on all cylinders?

Are the timing marks lined up on the engine and within the mag?

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John *.?-!.* cub owner
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Re: Mag issues

Postby John *.?-!.* cub owner » Sat May 14, 2016 9:20 pm

ScottyD'sdad wrote:I have a power unit on my cordwood saw, with a mag on it. No problem, starting it with 12 volts.
Ed
As I stated, the problem of impulse not working with 12 volts is rare.
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Re: Mag issues

Postby Jim Becker » Sat May 14, 2016 9:48 pm

John *.?-!.* cub owner wrote:
ScottyD'sdad wrote:I have a power unit on my cordwood saw, with a mag on it. No problem, starting it with 12 volts.
Ed
As I stated, the problem of impulse not working with 12 volts is rare.

For one thing, it depends on just how bad the starter and wiring got before somebody "fixed it" with a 12-volt battery. If the stuff is really shot, it won't spin any faster than it did originally on 6 volts. Low compression will make a difference too.


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