1987 Other Chevrolet Models BACKFIRES AND CARB FLOODING

IT'S AN RV CHEVY MOTORHOME. IM DRIVING DOWN THE ROAD WITH NO PROBLEMS AT 65 MILES AN HOUR I BEGIN TO SLOW DOWN UP AHEAD IS MY EXIT. AS I START TO SLOW DOWN IT BACK FIRES AND THEN STALLS OUT AROUND THE TURN. I LET IT COOL DOWN FOR A COUPLE OF MINS, AND TRIED STARTING AGAIN THE GAS WAS STARTING TO SMELL HEAVY, I GOT IT STARTED BY HOLDING DOWN THE PEDDLE AND IF I TOOK MY FOOT OFF IT WILL STALL AGAIN, SO I HAVE TOO KEEP MY FOOT ON THE GAS PEDDLE TO KEEP IT RUNNING, NOW AS IM DRIVING I CAN SEE THE GAS SPITTING UP THROUGH THE MIDDLE OF THE FLOAT VENT AND THE TWO FRONT FLAPS OF THE CARB. ITS A FOUR BBL QUAD-RODCHESTER, ELECT CHOKE, NO ECM PLUG INS, JUST A PLAIN 4BBL CARB. IS THE FLOODING BECAUSE OF THE MUFFLERS IS BAD CLASPED OR BAD CARB. THE SMOKE IS BLACK OUT TAIL PIPES. I DID A COMPLETE TUNE UP 2 MONTHS AGO. 1987 - 350, 5.7 L GAS MINNIE WINNIE MOTOR HOME 24' I CAN GET IT TO DRIVE UP TO 40-45 MILES A HOUR AND THEN IT WILL STALL ANY THING OVER THAT.