Running Rich

Did a routine spark plug change today and noticed an unusually high level of black deposits on the old plugs.  Since I bought the car a year ago it had that rich running smell about it when the bonnet was opened.  In the old days you would get the car plugged into a Crypton tuner to set up the carb.  Is this still the case or could I just tweek down the mixture screw (wherever it is) a little at a time?  Any advice would be appreciated.

If you've got access to a crypton, use it… I bought a Gunson Gastester - after a few uses, it's paid for itself.

Which carb have you got? The mixture screw on a Weber DMTL is down a small tube, near the front lower corner on the driver's side, and is a slotted head screw.