Drive belt to Bobbin is OK, but vertical shaft only turns part way then stops. Result---the rest of machine functions but the bobbin doesn't. Machine had not been used for a few years. Stored in dry area,

Gordon, October 2007

Hi!That was the correct answer to your problem. Your machine has a broken gear. For the gear and how to replace it...go to
http://www.tandtrepair.com/StylistLine.html
Hope this helps. Bill Eaton, old Singer man.

Bill Eaton, December 2007

Hello Gordon,
I have a feeling that the top nylon gear is split, this is a common fault on all 400 models, the nylon gear drys out over the years and breaks up, take the top cover off and round about the middle of the machine you will see the two nylon gears, look at the vertical gear, you may see the teeth broken away, the gear will probably look dark yellow now, you can get the gears and you can fit it yourself if you are a bit handy.
Ken

drivewithken@blueyonder.co.uk, October 2007