Tranny Vacuum Modulator

The white stripe is for the C4s up to 72. The yellow stripe modulators are for late 72 and on - they also replace the dual diaphram modulators.
Turning the adjustment screw will do nothing for full throttle shift rpm.
The adjustment is for part throttle shifts. The problem is that you can only turn them 2 to four turns maximum in either directins without causing slow death to your transmission. When you make the modulator pressures shift later you are making the transmission shift when the engine is producing more hp and the transmission pressures are dropping. You will have later but poorer shifts with bands and clutches sliding together instead of locking up quickly and efficiently at the lower rpm.
You can modify the governor primary valve to assist in later firm shifts but it also affects your milage.
Your transmission is supposed to shift at a low rpm when you don't have your foot in it. If you are trying to change the full throttle shift point the modulator won't do anything for that. Full throttle shifts are controlled by the governor and internal (in some valve bodies) pressure modification valving. You can replace the governor and get 5800 rpm shifts under full throttle safely or you can get a manual valve and shift whenever you want.