To check if tension is out of wack - sew a straight line on the BIAS/Diaginal of a piece of fabric. Then gently pull the stitching by holding both ends. If nothing breaks then the tesnions are equal. If the top thread breaks then the top is too tight and if the bottom breaks then the bottom is too tight. Adjust and try again. IMHO you need to try changing several things- Make sure both bobbin and top thread are the same, Make sure your presser foot is up when threading top thread since a having the foot down will close the upper tension disks causing the upper tension to be off., rethread both bobbin and top.,Check stitch length. Last resort take to repair shop. I NEVER mess with the screw on the bobbin since most tensiuon issues can be fixed other ways.