How do I adjust the fuel mixture on my 1987 Dodge w-150 pickup?

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My Dodge pickup has a 318 with a Holley 2 barrel carb. Due to black smoke when I start it I think and have been told that my pickup is running too rich. How do I adjust this? I have looked at it, and can't seem to find the adusting screws. There is one screw on the front of the carb that has a spring over it. I can screw this screw in and out and the first time I did it the motor would run faster or slower depending on which way I turned it. I adjusted it and the pickup seemed to run better for a while with no smoke, but it's right back where it was again. Now when I turn the screw nothing happens. The motor runs the same. I have used carburetor cleaner, and have added cleaners to my gas but I still have smoke. I looked around on the carburetor but can't see any other screws I can adjust. Maybe someone could help me so I can avoid taking the pickup to the mechanic. thanks