Carburetor

We have a ChevyG20 van. When we bought it ran great. The carburetor had been replaced with one from U Pull It. My old man has a habit of shutting off the van at lights to save gas. The last time he did it, the van would not start again. He changed the inline fuel filter. It did not work at all. We had to take the carburetor/air filter assembly apart and messed with the choke/butterfly. It started again, but since then we used carburetor/choke cleaner and changed the inline fuel filter again. Today I had to hold the butterfly open just to get in town. It keeps shutting. We used three gallons of gas to go twelve miles. It is blowing black smoke and bucking. When it gets into third gear it just will not go past 45. Please help!