Are these the engines that are supposedly run in before you buy them? Don't believe it, it'd take them longer to run them in than to make them. Always at least take off the backplate to check inside them before the first run and if you find anything inside, even metal dust, then thoroughly flush it all out. One tiny bit of aluminium dust getting on the plug is enough to kill the plug which sounds like what happened to yours. A plug can glow with the battery connected but not necessarily work without it.