When using an older machine, and by older I mean one that isn't electronic, bobbin tension is a very important thing to consider and something that you have to set manually. If the tension is not correct, it can lead to skipped stitches, bird-nesting (that incredible tangle on the bottom side of the fabric you are sewing) and stitches that looks puckered. The overall result is a garment or quilt that does not look its best.
What should the stitch look like? If you picture the fabric as a thin piece of plywood, the bobbin thread and the thread from the needle should be balanced. That is, the stitch should be constructed such that the thread from the bobbin stays on the bottom, only coming half way up the needle hole where it meets the needle thread and passes over it to hold it in place. Too much tension and the needle thread is visible on the bottom of the fabric and too little tension and the bobbin thread is visible on top of the fabric. For a thicker fabric, like denim, if the bobbin and needle thread tensions are balanced then you should be able to use a blue thread on top and a white thread in the bobbin and have no way to tell just by looking at the finished garment. For thin threads like Batiste, this really is not possible but the puckering or skipped/gappy stitch effects are more noticeable.
What do you do? Use a bobbin tension tester. For Singer Featherweight machines, the bobbin tension is ideally 23 grams, give or take a gram. The tension is adjustable on the bobbin case using a screwdriver. Simply put the threaded bobbin in the bobbin case and attach the thread tail to the clip on the meter. Gently pull the bobbin case until the thread starts to unwind. Then make adjustments to the bobbin case tension screw as needed. You can see this done here. As the Singer 99 shares a lot of parts with the featherweight, including the bobbin and case, I'm going to venture a guess that 23 grams or 4/5 of an ounce would also be a good tension for that machine.
Do different fabrics require bobbin tension adjustments? Yes they do. For these, you are going to have to look at a sample of the machine's stitches using a scrap of the fabric you will be sewing. When sewing with a very fine or very fabric or thick thread, a change to the bobbin thread tension may be necessary for the stitches to look right. If the bobbin thread keeps breaking when you sew, that is a clue that the bobbin tension may not be where it needs to be.
In closing,I want to pass along three web sites that I have used to buy items for both my featherweight and my 99. They are:
Machine manuals:
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