mine overfuels......no black smoke tho...... just big flames when reving it high. i cant see anyway of altering the fueling as this is done by ecu..... all's i can think of is the airflow meters worn and is fooling the ecu..... try resetting the ecu it may be scrammbled.......disconect the battery for 30mins reconect it and go for a drive reving through out the rev range so it can restore its parameters.
this may sort it..... you know how crap computers are crashing all the time well your ecu is the same......
i take it it doesnt need chipping!

also check your plugs as it may not be igniting properly...
you wouldnt get this problem on the cat converter cars as the ecu ajusts the mixture to prevent cat damage also helps the mixture to be bang on.....not too lean not too rich...just right.....