1989 GMC Jimmy

Engine Performance problem
1989 GMC Jimmy 6 cyl Four Wheel Drive Automatic

IT WILL START BUT WILL NOT STAY RUNNING BY ITSELF YOU HAVE TO KEEP PUMPING THE GAS PEDAL WHEN YOU LET OF GAS PEDAL IT STALLS OUT WE HAVE CHANGED VACUUM LINES CLEANED CARBURATOR BUTT STILL DOING SAME THING DON'T KNOW WHERE TO LOOK NEXT HAVE ONLY HAD THIS VEHICLE FOR 3 WEEKS WAS TOLD IT RUNS GOOD WHAT DO I CHECK NEXT