running rich and popping when slowing

[jrb92se]

i just got my car (rb25det 92 s13 coupe walbro 255 three inch blitz nur spec stock boost ) its seems to be running a little rich also when im slowing down at stop lights etc. it pops its not really loud like a backfire but you can hear it. is this normal my friend says its because of the turbo ive never had a boosted car till now so i dont know, also if i got one of those fmu or a nismo fpr would that help with the rich condition the cars got a walbro 255. also it kind of has a rumble in it it almost sounds like a ka is that normal do all nissans sound like that. is it common for rb25det crankshaft bearings to fail? i know your gonna say search but i did inthe rb tech forum for the most part and came back with nothing