Exactly what kind of modulator are you using? The small brass ones are made to be adjustable. The larger ones are not readily adjustable. They have sealant around the threaded stud at the end. Anytime you adjust that kind, you must reseal the threads or it will leak vacuum. In any case, the modulator only adjusts minimum and part throttle shift points (2-5 MPH), and to a minor degree, shift feel. Turning the screw in makes for later, firmer shifts, and turning the screw out makes for earlier softer shifts. Be careful with the adjustment when turning it out. It is possible to lower the transmission line pressure to the point where clutches slip and wear. If you want harder shifts, a shift kit is the answer. The modulator adjustment is limited.
98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54