need help badly on my 67 mustang coupe with 6 cylinder engine and auto tranny.. The engine would start but would not idle. i drive it once for a few miles.suddenly the engine splutter and spewd black smoke. it would not start since then. i've changed the spark plugs,condenser and contact point. i've checked the timing too. i then dismantle the carburetor(single barrel) and cleaned it. the gas tank was also cleaned. fuel pump and filter was replaced with a new one. it the starts and runs smoothly for a while. after a few miles of driving, the engine dies whenever i get my foot off the accelerator. it would start but would not idle. i have to step on the gas that hard just to keep the engine running. where or what do you think the problem is. should i buy a new carburetor. the mechanic said, the carb isn't giving enough gas. what should i do