if you're convinced you're running too rich, then yes either adjust your needle or deristrict your airflow. By lowering the needle you can lean out your mix a bit, but as far as i know this really only affects from 1/4 to 3/4 throttle. If you can find some jets, rejet. If you cant, then you'll need to drill holes or deristrict in some similar manner. aka take your airbox off and experiment with varying levels of pantyhose, laundry sheets, screen window mesh, performance filters, etc, to let the perfect amount of air in.
But what i dont get is how after putting on a biturbo you now run too rich... i was under the impression that expansion pipes required _more_ fuel, not less...