Black soot out of exhaust pipe

I have a stock 225 engine with a 1bbl carb (rebuilt) and it seems to run just fine but I noticed the back of the exhaust pipe is really black and sooty (that's a technical term). Anyway, it does not seem normal to me so I wonder what's wrong. I am soon going to add a super-six setup so it may be a non-issue but I'm still wondering what the fix is.