Getting the tension adjusted after you have had it apart can be tricky. Some times it needs an extra tweak just before you put the last nut on. Some times if it has more than one way to hold it on I adjust the tension then put the last nut on. If not some times you can push in on some of the parts and then give that last nut an extra turn. Then adjust. I suppose I'm speaking a foreign language though. I had a Singer 500 give me fits the other day. I just had to keep taking it apart and putting it back until I was happy with it. I think it has a lousy system. The 401 has an extra part and is so much easier to get it adjusted. Some times if you get one part in backwards the whole thing won't work at all. I don't know if it is always true, the nut that holds it on maybe should be about even with the shaft end. Maybe somebody knows better than I do. l Some times somebody else was there and buggered it before you ever got your hands on it - probably why they let it go.