adding lead would be the last thing i would do. Make sure you have your CG as described on directions and make sure you balance it out from near the fuselage and not on wing tips.

try moving your battery up as far as ya can to nose and any other gear to the nose as possible. Also make sure you are balancing with an empty fuel tank.

Adding lead may make the plane balance correctly but it should be the last thing you do. You can always slide your engine to the most forward position on the engine mount. Move the engine as far as it will go and see if that works also.

Try to see if any of these ideas work out for you before adding weight.