1991 Chevy Camaro 1991 camaro rs

i have a 1991 camaro rs 5.0 carburated tbi the problem is that when i turn the car on the car sounds real smooth and nice a bit of raw gas smell thats pretty much it the problem starts when i put it on drive it sometimes drives really rough when i give it gas it might not move at all and sounds like if something pops under the car like a fart or it may run but real rough like forced to run. it runs really nice in the highway at high speeds the problem is in the streets. i have replaced the waterpump,gaspump,radiator,full tune up, it doesnt have a catylic converter i took it out put a straight pipe, it does say service engine soon so please help i will really appreciated thanks.