check the idle mixture first! You get the factory setting by completely turning clockwise the mixture screw the turning it back (conter-clockwise) by 1 1/2 turns. You should start from this but you can further fine-tune the idle mixture (too complex to explain...). Be sure also, that the idle speed is not too low! Even with new spark plug and a perfect idle mixture setting, your beloved XL won´t start. Set the idle at 1200rpm at least! Bye!