black smoke ,1987 chevy siburban v 20 4X4,2000 miles on new

my 87 suburban blows black smoke n hesitates usually when going up hill at high speeds{65 to plus mph]gas milage is 6mpg copaired to the usual 13 to 15.i changed the throttle body,removed the cat.,changed 02sensor,changed fuel filter,all tune up parts are new.check engine code reads rich burn.this is a 1987 suburban with a 350,throttle body injection.original milage is 102,000,but only 2000 on new engine