Runs lean 1/4 tank of fuel and under

[ppl96zen]

ok, so i have a s14 and my current setup is Sr20 redtop completely stock motor running on stock 7psi and stock factory ecu, FMIC, and walbro 255 fuel pump. So ever since i did the swap ive been noticing that when i get down to about between 1/2 and 1/4 tank of gas my wideband starts showing very lean AFRs. with a full tank during WOT the AFR stays around 13.3 to 13.8, but when i have about 1/4 tank or less the WOT AFRs for the same RPM and boost in the same gear will be between 15.5 to 16.0. and basically Im just wondering if anyone else on here has had this problem before.