Fueling & Injection - Max psi without a reg, Walbro GSL392?

Just wanting to know what max psi a 255LPH GSL392 pump would have if the reg wasnt hooked up?

Reason being is I just want to test fire the engine for a few minutes getting everything going as it may sit another 3 months.

If you have an open return without a regulator the pressure will be very low without the regulator as a restriction. It will make the tune off....

Will it run at all?