I change tension on the top and bobbin when I change thread. The type, weight, and ply of the thread makes a difference in tension to get a balanced stitch. All machines are set to use poly thread at the factory. Notice the thread type used at the dealers on the set up machines. One dealer refused to put Aurifil thread in a machine I was interested in unless I bought the spool and machine. She would have had to get the tech to set the tension differently. ? I bought elsewhere.