My boiler pilot light lights, but the safety valve will not open to ignite the boiler.?

The problem is more than likely your thermocouple. It resembles a sharp metal probe with the tip in the flame of the pilot light. When working correctly, it will send a signal to the burners valve saying" the pilot's on, it's safe to open the valve and give it gas". When covered with carbon, it will not perform, so take some sandpaper to it or unscrew it from the valve(it is connected by a copper line about 1/8 inch thick). Any home improvement store has a generic one for about $10. Hope this helps.