Singer 99 Series, Service Manual

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To Adjust the Thread Take-Up Spring
The thread take-up spring (T, Fig. 9) should be set so that when the eye of
the needle reaches the goods on the downward stroke of the needle bar, the
spring will be through acting and will rest against the stop on the thread take-
up spring regulator (U, Fig. 9). If the thread take-up spring is not correctly set,
loosen the set screw (R, Fig. 9) in the arm of the machine and turn the tension
stud (S, Fig. 9) to the right for more movement of the spring or to the left for
less movement. When the spring is correctly set, securely tighten the set screw
(R).
The tension on the thread take-up spring should be just sufficient to take up
the slack of the needle thread until the eye of the needle reaches the goods on
its decent.
To increase the tension on the thread take-up spring (T), loosen the tension
screw stud (S) and force the take-up spring from the recess in the regulator (U)
to the right between the regulator and the tension discs until the required
tension is obtained, then securely tighten the tension screw stud and force the
spring back into its position in the regulator recess. To decrease the tension,
force the spring to the left between the regulator and the tension discs.