Johnson TN-28 Mag Issues

[Buccaneer]

I’ve been trying to get the bugs out of this TN-28 that I went through this winter,
but I think it’s got me beat.
Three times now, I got it started in the barrel and it died for one reason or another,
and when I tried to restart it, the mag plate is stuck to a coil lamination and it
retards the spark.

The first time it died because one of the plug wires got ate.
I replaced the plug wire this morning, checked the coil mount positions with a straight edge,
a.k.a., a machinist rule. I installed the flywheel, felt no rubbing when I turned the flywheel,
so I torqued it down, and still no rubbing.

I started the motor up in neutral, tuned a while, then went to shift it out of neutral and it died.
Went to pull the starter cord, and mag plate rotates with the flywheel again.
Pulled it apart, and could tell it’s one coil that was rubbing slightly on the center and one end lam.
I filed down the slight rub marks, and attempted to move the coils in a little, checking again
with my machinist rule. I couldn’t get the coil to move much, but it looked good with the rule.
I marked the lam ends with a magic marker, put the flywheel on and rotated it. Removed
flywheel, and seen no evidence of any rubbing. Torqued flywheel down, and still felt good.

This time I restarted the motor in gear, and ran it about 15 minutes, tuning, etc., trying
to idle down slower. It eventually stalled when I got to slow. I went to restart it,
and sure enough, the coil is rubbing on the flywheel again.
At least the plug wires didn’t get “eaten” by the flywheel again.

The motor has the good “green” German made OMC universal coils.
I may be able to move the coil in further if I take the plug wire grommet off that goes
thru the mag plate.
It’s just strange and confusing that there’s no evidence of the coil rubbing on the flywheel
until after the motor has been run, and “dies”.
No noticeable slop in the flywheel nor mag plate.
Any thoughts on why this is happening?

https://youtu.be/T1pc4e0kb_8

Prepare to be boarded!

[aquasonic]

Could it be that the coil is sliding just a bit once the motor is running? I had that problem with one of the TD horseshoe coils. Star lock washers under the coil mount screws fixed it.

Improvise-Adapt-Overcome

[Tom]

Did you look at the magnets to see whether there was a stray piece of metal in there?  Or loose on the mag plate?

T

[Buccaneer]

The mag plate seems to be plenty snug, and in the several times I had it apart,
I found no stray metal in the flywheel, or mag plate.
The coils bolts were snugged down good, so I can’t imagine they’re moving.
The last time I adjusted the coil positions, I put the problem lam “in” about .005
further than my straight edge indicated necessary.
The motor is on “Time Out” as it was a bad boy for not start last time!

Prepare to be boarded!

[billw]

I had a ’55 15 Evinrude that I had some weird coil rubbing issues with. It turned out to be that the crank bearings and bushings were worn out. When I rocked the flywheel side to side, left to right, fore and aft, it moved a bit more than “normal,” and you could hear an ever-so-slight, “tic-tock.” For what ever reason, it seemed to be worse when it had run for a few minutes, like yours. Maybe your upper and/or lower bushing is worn out? It might also explain what seems like some difficulty in keeping it tuned?

Long live American manufacturing!

• This reply was modified 3 years, 8 months ago by billw.

[Buccaneer]

Bill, I felt no side play in the crankshaft when checked, as I had the same thought.
Right now it’s staying on the rack. I do have one more TN parts motor that feels
like it has more compression, that I might do some parts swapping with some day.

Prepare to be boarded!

[Author]

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