Classic/Antique Car Repair: Carbon, adjustment screws, throttle application

Question
Hey Roger
Thanks so much for your previous help with my 64 Pontiac 230 6 cyl.standard.
Another question?

After I cold start it,there is a noticeable smudge of black carbon on the ground under the tail pipe.Is this from carbon buildup,and if so,do you recommend an additive to clean this,or could it be a carb problem?
Speaking of carbs,on the one barrell carb,there are two adjustment screws.One is connected to the trottle and is for the cub idle,but what is the other one,one the other side,for?
Also,when I am driving off from first gear,is it normal to give it a little gas while I am letting the clutch out? I feel the idle is too fast because it takes off by itself when I let the clutch out without any gas/throttle application. Thanks!

Answer
Barry;
 The carbon is no big deal as long as it is only when you are starting it up. and a can of gas treatment would not hurt. The carb. the 2 screws - 1 is for low idle  and the 2nd is for the hi idle , if it is to fast turn the idle down a bit   ok