There is no adjustment on the plum back. The only adjustment available is to go with the lower power green spring or cutting the spring. I had to cut my spring in half in order to get the piston to open under vacuum. The TS plum back gave me a headache for a while. I was hearing a lot of flutter under heavy acceleration and I'm running a stock air box. I first noticed that the piston was not moving at all when hot. Sure, it moved freely on my bench when at room temp but not when hot and attached to the turbo. Even with the spring off, the piston would not move once the valve heated up. As a note, this was using the TS machine oil. The tolerances on the piston are too tight, and add to that, the slightest uneven or over tightening of the three fasteners were contributors. I first had to use some fine steel wool, a drill with a large Allen socket and some metal polish to slightly reduce the diameter of the piston to allow it to move when hot. The next issue was the spring tension was too great. You can't see the movement of the brass piston on the plum back with the unit mounted. To solve this, I reinstalled the stocker and placed the TS unit on my cowl with the vacuum line connected and drove around to see when and how fast the unit was opening and closing. After cutting numerous coils off of the softer green spring and adding a couple washers to fine tune, it works as advertised. It was a pain to get it working properly, but worth it. I no longer experience any part throttle rapid opening and closing of the electric OE unit. It also holds 25psi with no problem.