First, replace your needle with a new one, it is probably not very sharp anymore causing the imbalance, re-thread the machine, sew a test strip. To adjust the bobbin thread, remove the bobbin casing. Locate the screw on the side of the casing. Mark with a sharpie pen where the screw lines up on the casing. You can adjust the bobbin tension by moving the screw left or right. Sew another test strip, mark on your leather where the upper and lower tension is set for reference. Keep adjusting until you get it balanced. Also, there are many different leather needles, some punch the stitch hole and some pierce, I suggest purchasing several different styles and testing them on your leather. SuperiorThreads.com has great customer service, I have always ordered from them because they are very knowledgeable with all types of sewing machines and fabrics. When I am sewing leather, I usually replace the needle after each project, if I am sewing through glue and leather I replace it more often.
