Yep, I have had to loosen the plug and then spin with starter. It would start and run....tightening the plug with engine running is dangerous even with a shortened open-end wrench.
View this before you get your fingers and a wrench on the plug: http://www.piratas-aereos.netfirms.com/accidentes.htm NOT A PRETTY BUNCH OF PICS.

It too big of a pain doing that with the plane on a stand with a helper. Besides I strive to have them reliable and function properly. It now has a CH Ign on it.