Absolutely mystifying failure, 850 Commando Mk III

Having busy day so can't answer in detail but I had a similar set of symptoms including swapping plugs over. It was the trigger unit on one side. You see spark on both plugs but one is the wasted spark and the firing spark for that cylinder is not working.

Time for a Trispark. Graham

dobba99 said:

Had similar when called to look at a commando that displayed similar symptoms. The owner had sleeved the coils with a thin rubber sleeve so the earth path was compromised.

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Earth path?? Ignition coils are essentially step up transformers. Wire wrapped around iron cores. I have seen coil housing crushed by the clamps and resulting in the housing shorting out the coil, but normally the coil housing is not grounded. The rubber sleeve might have solved that issue.