XJ500 "Cafe" Build story :)

Not sure if I can find an XJ500 head around. Although I did a search just now and found something for an '89 XJ500 Maxim? Anyone ever heard of that? It's in Romania, not far from me actually.
https://bnb4moto.com/en/moto-parts-oem/930-cylinder-head.html

I did try swapping the connectors, I know for sure it's the coil. However, it could be my error, as this is the second time I don't think they'll replace it for free again. Especially if they found out I'm putting it on an xj and not a cb.

 

I dont know if it does. There are others that will know if it will fit your motor.

 

Why a coil might go bad:

On the secondary side there is an open in the plug wire or cap that creates a larger air gap than the internal insulation within the coil and the high voltage finds a path to ground through the internal coil insulation. The result is the coil enameled insulation is burned and eventually breaks down and consequently becomes a lower resistance path to ground even after the above is corrected. Did you replace the plug wires and caps with the new coils? Did you ohm the secondary to the metal bar as Polock suggested?

On the primary side the TCI is misbehaving. First, if an increase in dwell occurs the effective on time of the coil becomes excessive and damages the primary winding from overheating. I don't know for sure but I suspect this would show up on the meter. The earlier measurement you made was 2.5 ohms, which is right at spec, but typically they read just a bit higher. Compare that reading with the working coil. The second issue with a TCI that can occur is the safe mode for when the bike is not running and the key is on. This can easily be checked by measuring the voltage on the gray and orange wires when the key is set to run at either the coil or TCI. One of those two should power up low, and then approximately 2 to 3 seconds later it should switch to 12V. Whether it is the orange or gray wire powering up low is dependent upon the position of the reluctor, however whichever one does power up low should produce a spark on the applicable plugs when it switches to 12V.

When you said the coil was warm was it like hot to the touch and obviously warmer than the other?

Replacing the head or even pulling it to repair damaged spark plug threads seems extreme. You can find all sorts of examples on here where helicals are installed but taking precautions to ensure the swarf does not drop down into the combustion chamber.

I would never run an engine with a loose plug as depending on how bad it is air can be pulled into the combustion chamber causing it to run excessively lean.

 

I wouldnt take the risk of putting in a helical into the spark plug hole without pulling the head off. if the debri gets in between the rings it can damage the wall. Take it to a machine shop to do it to make sure it goes in straight and piece of mind.