I don't see where it is designed to be adjustable. But, from examining the sesign, I extect you douls slip another steel washer (spring pad, pn 10.107.107) below the spring and raise the honking pressure. I am not sure how much of a change that will make. But before you do, you might want to rethink the reason you want to do this and the likely side effect. Mine honk, like yours, around 400 psi. But when the pressure drops far enough, it stops honking and stops delivering usable air through the primary. On mine, that seems to occur between 100 and 200 psi. The secondary will continue to deliver air. If you make the modification, I expect the stop pressure will also be increased. BTW, this stop of the breathable gas occurs when the stem pulls back so far that the oscillator stops oscillating and seat firmly on the outlet seat. The air flow does not totally stop as the seat is not perfect but the amount of the flow is so small as to be unusable. So, maybe 400 is something best lived with. It is not bat as a penultimate warning of a need to surface.
