Will the big spring only change cut off pressure?
It is intended affect both equally. You would usually want a 20 PSI differential.
Here are some notes:
To raise or lower the cut-in and cut-out settings while keeping the
differential between those two settings constant, adjust the range
nut. The range nut is the 3/8-inch nut that adjusts the larger of
the two springs in Models FSG, FYG, FRG, and Type G Pumptrol
switches.
Turn the range nut clockwise to increase the cut-in pressure and
counter-clockwise to lower the cut-in pressure. Three and a half
revolutions of the range nut will change both the cut-in and
cut-out settings by approximately 10 psi.
Adjust the differential nut if you want to raise or lower the
cut-out setting while keeping the cut-in pressure constant. The
differential nut is the 3/8-inch nut that adjusts the smaller of
the two springs in Models FSG, FYG, FRG, and Type G switches. Turn
the differential nut clockwise to increase the cut-out pressure and
counter-clockwise to lower the cut-out pressure. Adjusting the
differential nut will change only the cut-out setting while the
cut-in setting remains unchanged.