Bottom Tension Adjustment
Under normal circumstances the bottom tension does not change in use, so once set correctly it should not have to be checked again. The need to adjust the bottom tension therefore mainly applies only to second hand machines you are not used to, or if the bobbin holder is changed for some reason.
The adjustment principle of the bottom tension is similar for all Singer machines whether they use the vibrating shuttle boat type bobbin case (VS, 28, 128 etc.), the horizontally mounted removable bobbin case (15, 221, 222, 301) or the oscillating shuttle carrier with drop in bobbins (66, 99, 201 etc). In all cases the tension is provided by a small bent metal spring which traps the thread between the body of the bobbin case/carrier and the spring. The spring is slightly arched with a small adjusting screw that controls the pressure of the spring on the body and thus how much pull is required for the thread to slip between the two.
Lower Tension Adjusting Screw
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If you have a spring balance, it is simply a matter of removing the bobbin holder, inserting and threading up a bobbin and then pulling on the end of the thread in the normal direction with the spring balance. The target figure should be around 30 to 40g. (1-1/4 to 1-1/2 oz) but it does not have to be too precise. However most people don’t have a spring balance handy, so alternatively you can tie a weight of the correct amount onto the thread and then jerk the bobbin gently upwards. This should cause 2 to 5 cm (1' to 2') of thread to slide through, but not for it to unwind freely.
Singer 140q Sewing Machine Manual
If you don't have a set of small weights, then you can use digital scales to weigh sugar, sand etc into a small ziplock bag to use instead.
Singer 140q Sewing Machine Manual Instructions
If the tension needs to be adjusted then it is simply a matter of tightening the screw near the centre of the spring to increase the tension, or loosening it to decrease the tension. Normally only slight movements are needed and so 1/16 to 1/8 of a turn should be ample steps before retesting.