Amal Premier - Casting Porosity?

marshg246 said:

To do it correctly per Tri-Spark you would shut the engine off, pull the wire, leave the plug in place, plug in a spare spark plug and lay it on the head. Then start it. That's is certainly the safest way and as a dealer I suppose I shouldn't say another way. The way I wrote is how I do it though.

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Fast Eddie said:

I thought the issue was only when the spark has nowhere to go? So, with a wasted spark ign, so long as one plugs is sparking nicely, all should be well. I think…

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https://www.accessnorton.com/NortonCommando/pazon-sure-fire-ignition-misfire.5415/post-58300

Good correction LAB. I was thinking about my set up, with a twin outlet single coil. Of course, with stock dual coils this should not be done.

marshg246 said:

If running without a plug were going to damage a coil, it would happen with points as well.

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And it does happen, even with points. But the bigger danger is that an unloaded secondary can cause an inductive "kick back" that can damage the switching transistor in an EI. I would never run an electronic ignition without a plug. YMMV of course.

marshg246 said:

Points or an EI are at ground most of the time - they are only open for the dwell time.

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Nope. The coil is "off" most of the time. Dwell time is the time the points (or transistor) is on.

marshg246 said:

If running without a plug were going to damage a coil, it would happen with points as well.

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According to norbsa48503 (Greg Fauth http://decentcycles.com/) the "manual" is the factory manual which obviously refers to points ignition:
"If the spark plug can’t provide the ground than it will find its own. The spark will go right through the side of the coil body to find this ground. I would suspect that the coil has been damaged. Sometimes the manual is wrong and not talking about a ground wire for this tuning crutch is just wrong. Power will always find a path to ground you can’t stop that. Now get a black marker and cross this bad advice out of your manual or add the bit about the wire with clips."

..., therefore, whether points or EI, pulling a plug cap off a running engine in my opinion is to be avoided.