1974 Johnson 25 25R74M Ignition

[mercurymark]

Hi All,

I recently acquired a 1974 25 horse Johnson. Being an old Mercury guy for the last 20 years, I wasn't too discerning (dumb in other words) when it came to checking the motor out. It appears that the whole ignition system was changed out with an older vintage. Recoil, flywheel, coils, the whole works. Is there any reason this wouldn?t run satisfactorily provided coils, points, condensers, etc are all in good shape? I did see it run in a barrel and it seemed OK. Luckily, I don't have much invested.

Thanks for whatever advice or knowledge you can provide.

Mark

 

[flyingscott]

Where are the coils located on it. Or is there a silver disc on the powerhead with the model # could be an older powerhead.

 

[mercurymark]

The tag on the power head and the SN tag match. The coils are under the flywheel.

 

[F_R]

No Title

It should look like this under the flywheel. The main spark coils are outside. But somebody may have changed it over to the older OMC Universal Magneto

 

[F_R]

No Title

For your reference, this is an OMC Universal Magneto with junk (cracked) coils. Is this the kind that is on your motor?

 

[F_R]

You asked if the older mag would run ok. YES!! That would be why somebody changed it. The original Lo-Tension mag didn't have many people in love with it.

 

[mercurymark]

Hi F_R,

I've not had the flywheel off, but there are no external coils. Because the engine is running, I believe it must be set up as in your second picture. I don't know if there are any advantages to the factory coil setup. I could find the parts to switch it back, but I'm thinking why bother.

Mark

 

[flyingscott]

I wouldn't bother changing it back it is a good set-up that works well. The low tension magneto was problematic more so on the 15 hp motors than the 25s but no need to change it back.

 

[mercurymark]

You guys have been a big help. Another question I have is what plug should I use? The J4J?

Thanks

 

[Acton]

I've got a 1975 25 HP Johnson 25R75B that uses the low tension magneto system. The flywheel on the low tension magneto system has two magnets in it, the older points and condenser system only runs with one magnet in the flywheel. Trying to get your hands on the flywheel with two magnets in it is a pain in the ass, I know because I just went through it. If I knew converting back to the old system was easy I would have done that. I hear it's also possible to convert these motors to the new CDI ignition system too.

 

[mercurymark]

Acton, I think I'm going to leave it as is unless it gives me trouble. But, from what I read, the old style works fine.

 

[flyingscott]

I've got a 1975 25 HP Johnson 25R75B that uses the low tension magneto system. The flywheel on the low tension magneto system has two magnets in it, the older points and condenser system only runs with one magnet in the flywheel. Trying to get your hands on the flywheel with two magnets in it is a pain in the ***, I know because I just went through it. If I knew converting back to the old system was easy I would have done that. I hear it's also possible to convert these motors to the new CDI ignition system too.

I do not believe you can convert that motor to a cdi ignition. The reason being it never came with one.