I just bought a '76 Camaro with a 305 engine. I have to have the gas pedal depressed about half way to get the engine to turn over and remain running. While it's running it runs rough. If I let up on the pedal, the engine dies. I changed all the vacuum hoses, and fuel pump but it still has the same problem. There is also a lot of light grey smoke from the exhaust. Any suggestions on what might be causing the problem?