The Bowden cable connects the throttle linkage with the transmission and delays the shifts as the pedal is pressed. You should only reach 20 mph in 4th at very light pedal. 1/2 pedal should delay the 3-4 shift until you are travelling about 35 mph.

Full pedal should delay the 3-4 shift until you are travelling about 50 mph (possibly higher on the '85).

If you are not meeting these numbers, then the cable is not tight enough. There is an adjustment for it under the hood. The cable housing sits in a bracket and the adjustment is right at the bracket. You want to tighten the cable a bit more so that more of the cable is pulled out of the housing.

Don't go crazy on the adjustment. It's fairly sensitive.