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Lifebeat wrote: . . . BUT if the spark grounding method could hurt my new coils, I'd rather know about it and avoid it... What is your take on this? . . .

Perhaps, but I have a difficult time imagining how. In a coil, the primary winding current builds an electromagnetic field. That field is shared by both the primary and secondary windings. When the points open the current in the primary winding goes to zero and the magnetic field that was built starts collapsing. But a collapsing as well as a rising magnetic field will induce a voltage in the coil's secondary windings. So in normal use the collapsing field builds a voltage in the secondary of such magnitude that that an arc will jump across the spark plug center electrode to the ground electrode. This gives the energy a path to dissipate and an arc will happen until the voltage is so low that it can't be maintained.

Now then, when you short a plug wire to ground for testing purposes, the difference to the above scenario is that there is no delay while the collapsing field builds enough voltage to arc the plug gap. The same amount of energy will be dissipated either way, with an arc across the plug electrodes or with the center electrode shorted to ground. So the same amount of energy, give or take, is dealt with, shorted or not shorted.

Now the situation is much different when, instead of shorting the plug wire to ground the plug wire is pulled off the plug to dangle in the air. Now the coil "sees" a much longer gap, a gap of, say, inches, compared to a few thousandths of inches across the plug electrodes. It would take a hell of a lot of voltage to bridge that air gap. That rise in voltage can rise higher than the electrical insulation of the coil windings can handle, and an internal arc happens in the coil body. This arc can burn windings in two or short adjacent windings together. Both of those conditions can be disasterous to a coil. Perhaps a coil can handle the much higher voltage and no harm is done. But that's a gamble you don't want to take.

All of which is to say you don't want to pull one plug wire to see how the other cylinder is doing. But mechanically shorting the plug wire is OK.


Ken

Using a spare spark plug laid on the fins is NOT the same as "pulling a wire" OR "Grounding".
You are only seeing if that particular side is actually operating the spark plug. The spare plug has the correct gap (I hope) and what the ignition system sees is electrically the same.
You do need to do the change with the engine off, however.