Johnson 15R74S Electrical issue

[Bullie]

I have reached the end of my understanding again. It happens often. I am working on a 1974 Johnson 15. It has a bright blue snapping spark on the bottom cylinder and an erratic and weak to no spark on the top. The top plug was black and oily looking and the bottom was brown as is usual.

What I did:
I removed and cleaned/polished the points on both sides, set them to the correct gap, replaced the condensers. No change. Figured that was the problem but no. I then switched the external coil for the top cylinder with an old one I know to be good. Nothing. I reinstalled the original and decided to swap the wires from the bottom coil to the top. Bam! Same bright blue spark I was getting on the bottom cylinder. So, I eliminated the coil as the issue. I removed the flywheel again and checked everything I had done and looked for any abrasions on the wiring that might cause a short to the top cylinder. I removed the plate and checked the clamp on top and bottom, no damage that I could find.

How does the driver coil alternate spark to the sides? Like I said, I have reached the end of my understanding and need some help. Is it likely the problem?

 

[gm280]

Have you swapped the points and condensers to see what happens? Also, look to see if there are any missing magnets on the flywheel. The exciter coil is needed to build up the current/voltage to charge the primary side of the spark coils. And when the points open, the field collapses on the primary side of the spark coil and that causes the secondary to fire a high voltage spark. Clean and tighten all the connections on all the components and not just think they're good. Remove them and make certain the connections are clean and shiny metal...

 

[Bullie]

I installed new condensers. I can swap the points and recheck.

So, if I understand you right, if the driver coil is providing spark to one cylinder it is not the problem?

 

[Bullie]

How can I test the driver coil?

 

[tomhath]

I reinstalled the original and decided to swap the wires from the bottom coil to the top. Bam! Same bright blue spark I was getting on the bottom cylinder.

If I understand that, you switched the plug wires and the good spark moved to the top? Sounds to me that you have a bad plug wire. The insulation does break down over time which allows the spark to short to the block instead of making it to the plug. New wires are cheap.

 

[Bullie]

I wasn't clear enough with that. I swapped the leads from the driver coil to the ignition coil in order to test the ignition coil on the non-firing cylinder. Originally, I had fire on the number 2 cylinder only. When I moved the number 2 lead to the number 1 coil I had spark there. I was trying to narrow down the issue. I assumed that eliminated the ignition coil as the problem.

 

[F_R]

You asked how the driver coil alternates the spark top to bottom. Simple. As the flywheel passes the driver coil, both sets of points are closed, which means both ends of the driver coil are grounded. When it is time for a cylinder to fire, one set of points opens and the other set remains closed. That means one end of the driver coil is grounded, but the circuit is broken on the other end. That makes the voltage in the driver rush out to the spark coil (on the ungrounded end).

Rotate the flywheel 180 degrees, and the same thing happens as the other set of points opens while the first set remains closed, and driver voltage rushes out to the other spark coil.

That is why BOTH sets of points must be in good condition. One set has to grounded, then open to send voltage to the spark coil, while the opposite side must be making good ground contact at the same time.

The mag plate must be grounded for the voltage to travel in a complete circuit from driver to the spark coil and back to the driver. There is a ground wire for that purpose. Is it there? And not corroded or broken?

 

[Bullie]

Alright. That helped me a lot. One side is functioning as it should. The # 1 cylinder has no fire. I assume I need to check for continuity from the breaker points for that cylinder to the connector for the ignition coil as I know the coil is functioning?

I haven't looked yet at the ground wire. But, would one side functioning properly indicate that the ground wire is intact or are there separate grounds?

 

[Bullie]

Update. I switched the points and there is no change. I have spark to the same cylinder. I checked continuity on the wires from the points to the ignition coils. They were the same. So, can one side of the driver coil go bad?

 

[Bullie]

I bought a new driver coil. Still the same issue. No spark on top.

 

[SpokeBreaker]

I seem to be having the exact same issue with the exact same motor. 1974 15hp Johnson. Mine is a electric start. #1 has very weak intermittent spark, #2 has a very strong loud bright blue spark when testing. with a spark tester set at 1/4". Seeing low power at full throttle.
Here is what I have tried on the spark side:
New NGK b6hs plugs at .030
New condensers
New points set to .020 (tried the multi meter way too.)
New BRP blue coil (made it worse, bought a different one, same issue, back to original green as it seemed slightly better.)

What's left? Driver coil? Flywheel?

 

[AlTn]

a member a few years back solved a similar problem with this type ignition by removing the armature plate, cleaning it to as new condition,and sanding all the mounting points

 

[Bullie]

Back on this motor again after a long break. I have done what many do when frustrated, I threw parts at it. It has new points, condensers, coils, and drive coil. It still has intermittent or no spark. I checked/cleaned the ground wire from the plate to ground. Advice is appreciated.

 

[thumbnut]

The flywheel magnets are N-S and then S-N. Check magnets and make sure it is the correct wheel.

 

[Bullie]

Upon rereading this thread, I noticed AITn had shared that another member had a similar issue and had solved it by removing the plate and all ignition components, cleaning everything up, and sanding all contacts. I wish I had noticed that post before buying all the new parts. It worked! I took everything off the plate, removed the coils, etc and used my dremel to shine every contact up. I put everything back together and now have spark jumping 7/16. Thanks AITn. Wish I had paid closer attention.

 

[Bullie]

Motor is running well now.