Mechanical fuel pump, broken?

a mech FP is not the first place i would look. it is hard for them to fail and perform better than the originally did.

If it is a Holley, make sure the power valve is not leaking fuel through the valve and, or vacuum through the diaphram, either problem will allow exta fuel full time.

Next item is check the float levels, see if one is sticking open.

Next, especially on vacuum secondary 4160 carbs, remove the rear bowl and tighten the six screws that hold the metering plate to the main body. If this fixes it, the rear bowl/metering plate gasket has become fuel saturated, and is leaking fuel into the secondarys. At some point, this will show up as a rich idle with black smoke.