Doing a google search for the brand plus model number for a user guide or YouTube videos might find what you are looking for. That it is a Necchi doesn't supply the model number and of course models differ. On a 537L model, the large knob on the right side about halfway between the top and bottom of the machine adjusts stitch length. The numbers may range from 0 to 5 with 0 meaning no length and 5 being the largest length. Tension, stitch length and stitch width are all controlled by small dials on the front of the Necchi machine. If your stitch length is set to 0, the sewing machine will not feed the fabric. Incorrect tension will cause poor stitch quality. You must consult you machine's user manual to know how to adjust the tension as the procedure differs among machines. Here is a general guide, however:
How to Adjust the Tension on Sewing Machinehttps://www.youtube.com/watch?v=i-7bnb_6EBU Necchi
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Suggestions include checking similar models for sale on the internet, including EBAY.COM and specialized sewing machine web sites.