Monty,
I have around 1/2 a gallon through my 21-7fms now and love the engine. Has great power all the way through. Had a problem yesterday though, the fillament on the 97t plug broke. While running it in my truggy something suddenly didn't sound rite. I pulled it off the track, pulled the plug and the fillament had broke up inside the plug and was dangling. I am crossing my fingers that none of the fillament actualy fell into the piston/sleeve. Going to tear it down today and find out. I have been running the engine around 225 deg. HSN a tic in from flush and about 1/2 turn out on LSN, still running a little fat. I guess I've been fortunate enough I have never had a fillament brake before. I think for peace of mind I will go back to the old faithfull c5tg or c6tg nova plugs. Turning it over by hand it still has great compression. As far as the enginge goes I love it, great power and idles well. Say a prayer for me.