They can be adjusted, but they are tricky to do.
Use a wrench to hold the upper nut, and Loosen the lower nut. Then tighten down the upper nut until the spring is 3/4" high on the top.
Adjusting the single cable can be more of an art form then a specific setting. So what you will have to do is play with the spring tension by adjusting the upper nut and lower nut. Typically, when the car jerks into gear, the spring is too loose due to the cable stretching.
The shifting is actually done by gears inside the differential. They look like 4 interlocking fingers (use your hands to see that
). The spring puts pressure on one of the gears to force the teeth to interlock and move the car. When there isn't enough pressure there, the gears will spin (sometimes making a weird grinding noise) until enough centrifugal force engages them... by that time the engine rpms are high enough that it really jerks.
Have a great one,