It's a bit more complicated than a simple screw setting. Getting the XPAG to perform at it's maximum (which isn't saying much) requires a methodical sequence of actions on all the parts controlling the combustion process. I highly recommend the tuning instructions in the New England MGT Register's superb bible on all things MG - "The "T"-Series MG Handbook".
Start with setting the timing, valve clearance, dist point and spark plug clearances before you touch the carbs. Then verify the carb oil dashpot is filled with SAE 20 oil to 1/2" of the top, the correct needles are set correctly in the piston, and the jets are centered. Screw the jet adjusting nut up until it stops, then back down one full turn, or 6 "flats", for an initial setting. Loosen the linkage connection between the carbs, then synchronize them - matching airflow using a flow metering device. Adjust the idle speed as desired. Finally, adjust the mixture - raise the jet nut to weaken the mixture, lower to richen.
There is a lengthly discussion on lifting the suction piston to verify a correct mixture in the book mentioned earlier. Or use one of those "see-through" spark plug kits, or best of all, an exhaust analyzer.
If you can't get the mixture to lean out enough, you've got an incorrectly installed and leaking upper cork gland washer in the affected jet assembly. Disassemble and try again, soaking the cork in oil for 24 hours before re-assembly.
Hope this helps ...