I put E 3 spark plugs in my fresh built 360. Ran it about 350 miles and checked the plugs and they were black. It ran ok but not the way I thought it should have. Has anyone used these plugs? I removed them and put NGK plugs in. It seems to run much better. I'm going to put another 350 or so miles on them and check these plugs. We set the timing and ajusted the carb. twice with the old plugs I hope this helps.