i just bought my camaro in the last two weeks and i have been working out some of the minor bugs. I talked to my uncle who is a motorcycle mechanic about it and he says that the fine black powder-like stuff coming out of my exhaust is carbon and is most likely due to the car running rich (keep in mind that the car has no cats). i know he is very knowledgeable and he says he has seen the exact things on a great deal of bikes that come in to his shop.
when i bought it also the kid i bought it from says it was getting 10-12 mpg, which also made me immediately think that it is running rich or there is some problem that needs to be fixed. it is a 1987 carburated 305 that is basically bone stock with the exception of aftermarket exhaust. i have also not driven it enough to know what feels "normal" on it.

is there any way that i can adjust how lean/rich it is running?
could there be a problem with my choke or something like that?

thanks in advance for your assistance. if you feel that this is posted in the wrong area, please tell me where it should be posted and i will happily post it there. thanks again.