GMC C2500 Engine Rich

My Truck Is Running Real Rich Where You Can Smell Gas In The Exhaust Which Also Is Real Black, And I Am Now Getting Like 7 Miles A Gallon. I...

If I Rev It Up To 2800 Rpm And Let It Fall To 1500 Rpm Then 2nd Will Engage And Continue Driving Normally. Fliud, Filter, And "a" And "b"...