what does closed loop fuelling do?

it looks at the reading from the lambda sensor and adjusts the amount of fuel going into the engine until that output is where it wants to be.

if you adjust the global multiplier to put in more fuel, this will result in the output from the lambda sensor going up so the closed loop controller will just reduce the amount of fuel again until it was where it was before and the lambda sensor reads what it did before.

there should be instructions with your chip to say where the pot should be. put in that position and if your emissions are wrong then you have a problem somewhere that needs properly diagnosing, not just try and twiddle some knobs and hope to fix it.