The parabolic adjustment involves properly aligning the cam gear to the upper shaft gear. As the cam gear is turned, the linkage of follower levers controls the movement of the needle bar.
To make the adjustment, locate the cam gear connection to the upper shaft. Notice the upper shaft gear has a collar with two or more set screws on it. Loosen the set screws. Turn the hand wheel slightly, and check the needle bar movement. Using trial and error, keep working the handwheel in small increments until the needle bar moves straight up and down and swings at the peak height.