The easiest way to get your threads adjusted and learn which thread does what is to thread the machine with 4 different colors. Set your tension dials at 4,3,2,1 from left to right. This is the starting point. Sew on a test strip and look at the threads. You can then adjust the correct tension dial according to the thread color. This is a great way to get the machine set back to correct tensions as well as a learning tool to figure out what each thread looks like so you can adjust later when using all one color.
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I want decrease the stich length, for the hem, but cannot do it
The easiest way to get your threads adjusted and learn which thread does what is to thread the machine with 4 different colors. Set your tension dials at 4,3,2,1 from left to right. This is the starting point. Sew on a test strip and look at the threads. You can then adjust the correct tension dial according to the thread color. This is a great way to get the machine set back to correct tensions as well as a learning tool to figure out what each thread looks like so you can adjust later when using all one color.