best for spark transfer; dull or sharp edges?

Rat Bastid said

Glad you posted this. If you have a weak ignition system, any fine wire plug can show an improvement. If it does, you are better off upgrading the ignition system than using a fine wire plug.

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Two motorcycles I used them on one had Lucas electrical parts (known as the Prince Of Darkness) and the other is an India made points ignition single cylinder Royal Enfield. Those two did show improvements as I mentioned, the others were modern multi cylinder bikes and I didn’t notice anything honestly