I rebuilt the carb, set the mixture screw to one turn out and it ran good. However, black smoke was coming out as it never did before under idle. So I turned the mixture screw in to lean it out. As I did the rpm's picked up but the idle didn't change much as far as roughness, it has always idled pretty smooth. I turned out the idle set to drop rpm's and the black smoke disappeared. No problem right, well it seems like the mixture screw is almost all the way in and now I fear I'm too lean. I need help so I don't do damage. Does this indicate a problem with the carb/vacuum?