Which stitch are you using?
To check thread tension for straight stitches, use different colors of thread in your top thread and bobbin. If the top thread shows on the bottom of the fabric, turn the dial counter-clockwise (towards a lower number). If the bottom thread shows on the top, turn the dial clockwise. Do not adjust the tension too much (for example, go from 2 to 2.5 if increasing the tension) at a time; test another set of stitches. Alternatively, you can adjust the bobbin tension in the reverse directions. However, it's very easy to damage the set screw in the bobbin case so do this as a last resort. Since this is a vertical bobbin case, put the bobbin in and hold onto the thread. If the thread moves when you dangle the bobbin case, tighten the screw. Loosen the screw if the thread doesn't pull out of the bobbin when you hold the unit.
For zig-zag decorative stitches lower the tension until the puckering stops. Usually the top thread will show on the bottom of the fabric.
If the tension is not changing when you turn the dial, make sure that the presser foot is down. The sewing machine may need repair. The tension mechanism depends on a spring and that can break. See the Singer site for a parts breakdown and contact your Singer dealer for the parts.
If the top and bottom fabric are not moving at the same rate, you can get a different type of puckering. You may want to get an even feed foot if there are several layers of fabric.
The manual for the 3116 is available from Singer: http://www.singerco.com/uploads/download/438_3116.pdf
I hope this helps.