96 chevy pickup4.3 wont start

Hello all i have a96 chevy full size with the 4.3 in it i changed the gas tank out because of a leak shortly after that i started getting the black smoke at idle and the flashing check engine light now it wont start i have spark and 70 lbs fuel pressure the used tank i got had verry little water in it so i put 2 bottles of stp gas treatment in dont know if that has anything to do with it

thank you
james

Hello all i have a96 chevy full size with the 4.3 in it i changed the gas tank out because of a leak shortly after that i started getting the black smoke at idle and the flashing check engine light now it wont start i have spark and 70 lbs fuel pressure the used tank i got had verry little water in it so i put 2 bottles of stp gas treatment in dont know if that has anything to do with it

thank you
james


take the spark plugs out and determine what's up .