The 9.1 reg is designed to vent if the pressure rises above what you've set the reg to.
It does this via a small o-ring/screw located in the center of the spring piston (which is visible through the regulator/velocity adjustment nut.
Some basic diagnosis questions:
What's your air source (Co2, 12gr, HPA @850PSI, etc)?
If it's the pressure relief screw, it's not really meant for adjustment but, you can try turning it in (it's a small allen screw accessible through the regulator adjustment nut) but, doing so can disable the overpressure protection if not done properly (which is less of a concern if you're using a fixed output HPA tank).
The best way to do it, is to tighten it fully, go to the chrono, adjust your velocity to 280 (or appropriate FPS). then start backing out the relief screw until it starts leaking slightly. Once it does, give it a 1/4-1/2 turn back in.
If that doesn't fix it, it may be the oring on the spring piston.
Degas the marker
Disassemble the marker (Check the videos in the Useful Resources thread)
Disassembled the reg and inspect the piston o-ring.