The Mercarb power circuit is used to add additional fuel to the main jet system when under power. As you throttle up, the power piston moved down opening the power jet. At idle the vacuum is at it's highest, which pulls up the power piston, closing the power jet.
The stem on the power jet and the power piston must be straight.
A frozen power piston could possibly cause too much fuel in the carburetor main circuit.
Watch a video about the Mercarb Power Circuit: