My 94 S15 Jimmy to much gas

Its a 94 s15 jimmy 4x4 4.3 vortech with 175000 miles on it I had it for one year I put a new fule pump, filter, new flowmaster exaust with no cat on it, new injector, new sending unit break booster, I was getting 360 miles to a tank now I am getting 270 to 280 to a tank no engine lights are on the only thing I have not done is plugs, cap, botton, any ideas would be helpfull thanks