[trularin]

If it is Black/Brown, you have fuel/mixture problems.

How old are the carb(s)?

Might be time for an overhaul.

What a lovely way to spend a week. Pull the carberators off the Cobra and rebuild them. NOT!

Check your plugs and see if any are sooty. If the rear bank of plugs are sooty, the rear carb may need a look at. If the fronts, then the front carb. If it is just one or two, try a tune-up with new wires.

Oh heck. After you are completely frustrated, just give the car to me ( I'll just take it off your hands to save you from this headache ) LOL.

Just my View.