Brand new carb-still problems, help?

Hi all,
my master mechanic installed the NOS 2 bbl carter 4958S (a 318 V8 version apparently) and also a NOS distributor in my Slant 225+automatic Dart today. Idles fine, acceleration is great BUT at certain rpm coughs and sputters just like all cylinders weren't firing or something. Loosening accelerator slightly brings it back to normal.
Previous carb+distributor combo had these symptoms too, anything from idle to higher rpms. Only felt like it started to run good only after 60 mph, then accelerated fine to 70 and up.

Plugs went really black. However the idle mixture is as lean as can be.

Any suggestions, anyone?