rough on one mag on 912ULS

You can do a resistance check on all four coils through the spark plug caps, it should read about 15K ohms. Check the the top front, top back, bottom front, bottom back. The circuit you will be measuring goes from one plug cap/connector through the plug wire, the coil, plug wire and out the other cap. Some times when pulling plug caps off the spark plug they get pulled away from the wire and or corrosion builds up in the caps connection to the wire. The resistance check is a quick way of determining an electrical issue.

The symptoms occurring at one particular frequency(RPM) indicates that something may be loose and shaking in sync at a particular harmonic frequency.

The Spark plug connectors just unscrew. Lefty, Loosey!

Try tightening each connector by one turn. Righty, Tighty.

If you find one that is particularly easy to turn, keep turning until it hits bottom.

If you have enough slack in the wiring, you can cut 1-2 wire diameters off of the spark plug cables to give the plug connectors a fresh bite on the wires.

Thanks, Garrett!  That sounds interesting and wonderfully concrete.  It speaks to me! :-)

I'll go back to the MMH wiring diagram, but can you clarify for me which pairs to test?  I think you said 1 & 3 top and 1 & 3 bottom, right?  And then 2/4 the same way?

Thanks again!

Thanks, Bill!  I like the idea of a harmonic vibration. We just haven't pinnd down anything yet :-/

I certainly can crank 'em down again.  I probably need to cut the zip ties, yes?  I'll go back to the MMH and check, but I'll take pointers on how hard to yank down the new ones if ya got 'em.

Thanks again!

It is as simple as the top front, top back and bottom front, bottom back. You can visually see/trace the plug wires to verify this.

Check out dielectric grease for the connections.

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