I don't think that cap is adjustable. It's a screw cap that I can loosen but it doesn't feel like it's supposed to be anything but screwed on tight. I did open up the effluent though after your recommendation and it seems to be producing a more consistent bubble flow now although still aggressively overflowing. I'm guessing at some point I should try slowly closing it off again until it stops overflowing?Sorry, thinking of my 160 which is the older version. mine has a PVC thumb screw to adjust the air. So the white cap on the silencer of the new model isn't adjustable? i was under the impression that cap turned to close off the air intake hole on the side.