Hello Gwen,
The tensions on the old black singer machine can be very temperamental, it really is trial and error, do you have a numberered tension? if so it should be set between 4 and 6, I take it that it is a round bobbin but is it a drop in bobbin or does it have a bobbin case that goes in the side, you say the machine is 25 years old +, if it is black it is more likely to be 45 years old or more especially if it does not have a model number underneath the stitch length, you can probably still get a manual for your machine, I have a list of sites, if it is possible for you to e-mail a picture or describe your machine in detail including the length of the bed of the machine I can probably pinpoint a model number.
Ken
drivewithken@blueyonder.co.uk, May 2008