Swapping low tension mag to universal magneto

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RCO
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Swapping low tension mag to universal magneto

Postby RCO » Thu May 31, 2018 4:12 pm

I have a 76 Johnson 25 with the low tension magneto that always seems to be acting up. Looking at the mag plate, it looks like I could just swap it with an older style universal magneto. Has anyone tried this, and will the original flywheel work?

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Chinewalker
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Re: Swapping low tension mag to universal magneto

Postby Chinewalker » Thu May 31, 2018 4:28 pm

You need the matching flywheel, too. The LT flywheel won't work with the standard magneto...

RCO
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Re: Swapping low tension mag to universal magneto

Postby RCO » Thu May 31, 2018 5:58 pm

Well, I got impatient and put the magneto from a QD I had in my parts bin on it. Switched over the cam and throttle link. Expecting I would have to change the flywheel, I decided to try it just for kicks with the original. It fired on the first pull and runs like a Swiss watch. There is two opposite pairs of magnets on the flywheel, so you end up with a waste spark ignition firing at the top and bottom of the stroke like most snowmobiles. I can't see this causing a problem, but I guess time will tell.

FrankR
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Re: Swapping low tension mag to universal magneto

Postby FrankR » Thu May 31, 2018 7:12 pm

Hey RCO, thanks for posting that. It answers a question I've chased around in my mind awhile. I've wondered if those magnets would cause a problem. Evidently not. Actually there shouldn't even be a wasted spark because the points opening would not be in time with the extra magnets going by.

RCO
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Re: Swapping low tension mag to universal magneto

Postby RCO » Thu May 31, 2018 8:31 pm

True, I didn't even think of that. You would only have a waste spark with one coil firing both plugs at the same time. I like the idea of having the same ignition as the rest of my OMC motors, and I can carry all the spare parts in my tackle box for under $50.

rudderless
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Re: Swapping low tension mag to universal magneto

Postby rudderless » Thu May 31, 2018 10:32 pm

I bet one or more plug fires from the ground to the center electrode...

Low tension mags have wheels with magnets which are n-s then s-n. See if the firing coil or plug sparks from ground to the center. Hopefully the right pair of mags are passing by the right coil at the right time. Sparking backwards is not a big deal just that the plug will grow gap quickly.
..any port in a storm..

FrankR
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Re: Swapping low tension mag to universal magneto

Postby FrankR » Fri Jun 01, 2018 8:54 am

Every OMC V-4 with magneto ignition has two positive and two negative sparks. Dang if I could ever tell any difference.

EDIT: Opposed twins also have one positive and one negative spark on their respective ends.

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20mercman
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Re: Swapping low tension mag to universal magneto

Postby 20mercman » Sun Jun 10, 2018 11:11 pm

I am working on a 7320R It is a 1973 20hp with this magneto. It keeps dropping a cylinder. Is this magneto one that will work if I try to retrofit onto this motor? Thanks for any help! https://www.ebay.com/itm/Johnson-Evinru ... SwBV5aUSbm

If so, would I only need the cam in addition to this plate and of course some coils?

Steve

RCO
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Re: Swapping low tension mag to universal magneto

Postby RCO » Mon Jun 11, 2018 12:07 pm

On that motor, why not just find a complete magneto for one of the older 22 cu motors? It should bolt right on without having to swap anything. I just used one from a 10 hp QD because I had it on hand. I would think you could find a direct fit with good coils and the correct length plug wires for under $50.

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20mercman
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Re: Swapping low tension mag to universal magneto

Postby 20mercman » Mon Jun 11, 2018 3:40 pm

RCO wrote:On that motor, why not just find a complete magneto for one of the older 22 cu motors? It should bolt right on without having to swap anything. I just used one from a 10 hp QD because I had it on hand. I would think you could find a direct fit with good coils and the correct length plug wires for under $50.



Thanks! I am a Mercury guy, so these OMC's are a bit out of my normal playground. Anyone have a lead on what I would need, it would be greatly appreciated.

Steve


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