Swapping low tension mag to universal magneto

[rco]

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?

[chinewalker]

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

[rco]

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]

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]

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]

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.

[frankr]

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.

[20mercman]

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]

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.

[20mercman]

quote RCO:

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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