912 ULS Engine Misfire

Looks like you found the problem. I wouldn't replace the coil if they tested good. Keep eye on the plug wires, especially where they pass under the intake manifold as any contact there can cause shorting out.

Ther coils very rarely ever go bad. , but rarely at the coil unless some kind of maintenace has been hard on thsoe connections. 

You can easily cut a short length off the HT leads and just screw them back into the coil, they certainly shouldn't just pull out like in your photo. There is also meant to be a length of heatshrink covering the nose on the coil. I cannot remember whether it is meant to be adhesive lined.

What I would say is that a failure of a single HT lead is unlikely to lead to an RPM drop as high as you report. That level of drop is more likely due to a complete ignition failure on one side. Have you tried switching off a single ignition when the misfire starts to see if you can isolate it? Have you also checked the resistance of the NGK boots fitted to the plugs? I cannot remember the resistance, I think that they were somewhere in the range 5-10k ohms. You can measure from one plug boot to its paired one through the coil, this will save you from taking the boots off the HT leads to make the measurements.

I would check things like the generator coils that supply the ignition modules. There are a couple of ground connections under the flywheel and there have been problems with the red supply leads on some engines. A loss of either supply will give you a fairly large RPM drop.

Good luck!

Thanks for the info! I will check those things and I'm taking it to the mechanic on Tuesday. They are going to look into the items you mentioned and I haven't tried isolating the misfire yet since it normally happens on takeoff (not the best time to switch off either side). I will try doing that on the ground to see if I can figure out which side is bad. I really appreciate the feedback and think that is more likely the cause as well based on the symptoms and your feedback. Will keep you posted! Thank you!

The high tension wires from the cold to the spark plug are a copper stranded type.  Be sure that you cut back the wire about 3 to 4 mm (1.8 inch at least) to get into a clean part of the wire.  Use a dielectric grease on the wire end also, do not install it dry.  This will help prevent corrosion that may occur over time.  The sleeve on the wire is for electrical suppression, it is simply a vinyl pull on peace and is not glued or stuck to the wire.  It is there to help reduce interference with radios and the like.  Let us know if this works for you. 

Cheers