appears to be black on the choke blade and air valve. indicator of lean condition or late timing; maybe both. I did a 4615 for a friend last summer who had a float issue. I think maybe those old floats may have seen there days. anyhow, I replaced the float (edelbrock floats will work), went to a .093" primary jet, and richened up the idle circuit using the adjusting needles at the base of the carb. works very well. the fuel evaporation issue is the nature of todays gas and engine heat. both my cars use carter carbs and I fill the float bowls thru the vents before starting if the car hasn't been ran regularly.