I was faced witht he exact same problem today. After changing the needle what seems like a thousand times. I chacked the plug, it would lite, fine out of the engine, but still I changed the plug just to make sure it was good, still same problem, so just for the fun of it, installed another new plug, then beleave it or not, All of the time it was 2 bad plugs, they would lite fine but would not stay hot under the load of the engine. Reset the needle back to where they should have been to start with and all is good.