miriam

It is super easy to take the whole thing off. There is a set screw inside the side door turn it loose and pull the tension out.
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It looks like one of the old stock replacement tensions - your problem may not be the tension at all - do you have one you can pull off another machine and test on there? I fought with one last winter. Not even a tension off another machine fixed it. There are springs that hold the bobbin holder in place and thread passes through there - if they are not set right you will not get a good stitch. The bobbin tension can also be set by tightening or loosening the spring on the bobbin holder. Some times those need to be cleaned out. I have also seen a tension post pin with so much dried up oil in there it didn't allow the tension pin to work freely. There could also be a burr or dried up oil somewhere on the thread path. The tensions that came on those machines are tricky to rebuild. There isn't a very clear manual pic of them either. You may be happier if you found a 401 tension to go on there if you aren't worried about how it looks. When you adjust the tension make it as loose as it will go. Then stitch. See how it looks, you can tighten it a little, check and tighten until you are happy with it. I wish I could help you rebuild one but it would be a stretch to do it through a computer screen.