I have witnessed Guinness pressure between 25 and 40 PSIG. Normally it should be between 28 and 35 based on the type of system. If you are dispensing from a kegerator (short system), start at 28 PSIG and increase from there. Check the restrictor palte in the spout for buildup as well.
Scott Zuhse, Instructor
Micro Matic Dispense Institute