did you take the plug out and put it in the igniter to make sure its glowing? if so the plug is good. next make sure there is an opening in the carb, take the air filter off and look inside, there should be a 1mm opening, if not you have to turn the idle screw clockwise to open it, its the one with the spring on it. if its cold out, you may need to warm the engine with a heat gun or hair dryer. but if you have glow, fuel and air, it should start.