lumpy idling,over fueling?
 

hi guys need a bit of advice. i have a year 2000 sti impreza. only have it a couple of months and all was running fine. in the past week its started idling badly. only when its up to running temprature. the rpm drops so low it almost dies. it all so seems to be over fueling as theres a bit of black smoke from the exhaust. thing is when i disconect the maf it all runs fine. would this mean thats what the problem is. i dont want to buy one till am sure as there expensive. the guy i bought it of told me he had replaced the maf and lamba sensor recently with used ones. he told me they were tested and running fine. i dont have a check engine light or anything till i disconect the maf. i also tried disconecting the battery to reset the ecu. this didnt improve anything. any help would be appreciated.

Sounds like maf.
Maf should only cost 90ish and on a 99/00 I would recommend one as a spare.
Therefore I would buy a new maf and try it, and keep old one as a spare if not the issue.