There is no middle speed needle, its an adjustable spraybar. Unless you are compensating for a tuned pipe, you should not touch it. It should be flush with the carb body. If you do decide to adjust it, you need to adjust it in conjunction with the LSN. If you back the spraybar out half a turn, you need to bring the LSN in half a turn. All the "MSN" does is let you play with the powerband a little. Look down in the carb, you see the needle that pokes into the little tube? The needle is the LSN, and that little tube is the spraybar, which is attached to the adjustment slot on the outside.