350 chevy motor qudra jet carb?

new exhaust on van when ya go down the road below 55 its fine but 60 to 75 it chugs spits and sputters backfires loses speed ya let off and its good to go again also some bad smell from exhaust not form being new but like a carbon or something smell has me stumped fuel filter is good all vac lines fine what coould be the problem and how do I fix it ?