Did you know?

There are idle ports fed from the secondary bowl on most 4150/60.
Instead of a needle valve, the port is sized as a fixed restriction.

Blockage in Holley Baseplate Passage

Exceptions are carbs where the 'snake passage' from the primary idle circuit is connected to the secondary idle port.
racingfuelsystems-Drilling secondary curb idle holes on a 2 corner adjustable carb