The only thing tinkering with the mixture screw accomplishes is changing the air/fuel ratio at idle. Once you get above 1200 RPM or so, the mixture needle could be bottomed out entirely and it would have no effect on the mixture at all. You've either got a very high float level, or a power valve stuck in the full rich position, or both. You could also have a float that's partially filled with gasoline and unable to exert enough force on the fuel inlet needle valve to stop the fuel flow when necessary.
Jerry
