I start my jeep, it turns over no problem, its a 6 cylinder 4.2 L from 87. once it starts, it runs fine for about a minute, then slowly sputters out, and when i press on the gas to keep it alive, it spits blackish grey smoke, and then i let it die. my dad and i think it is a vacuum leak, which is preventing air from getting into the engine block, and making the air fuel mixture too rich on fuel. I tried looking into the carburetor to see how the screws were adjusted, and they were adjusted just fine. I will note my dad when rebuilding the engine took out the idler air control valve, which may be causing this problem, but it ran fine without one for 2 years. I will also note that a tube connecting the air filter to the valve cover is loose, and there is a valve that is missing a cover, and has depressurized. there are 5 other of these little valves that all have covers on them and the one that is missing hasnt had one on there for a while.