One of the byproducts of all Hydro-Carbon combustion is...drum roll please...WATER. Until your exhaust gets hot, that water will transform from a gas (steam) to a liquid (water and water vapor). This liquid water will pick up all the carbon soot in your exhaust..esp carbed engines...and throw it out the back. What it does tell us, is that you have incomplete combustion during some part of your cycle...could be overly rich or not....my Dyno AFR's look pretty good..for making power, but I have a sooty tailpipe FWIW