The Mercarb accelerator pump circuit provides a little extra fuel when accelerating. Once the acceleration is over, the accelerator pump circuit no longer provides any fuel.
A hesitation when throttling up can be electrical, but often it is the accelerator pump circuit that has a problem.
Pump Rod Adjustment
The pump rod is adjusted so that the accelerator pump gets pushed down without any dead spots.
Testing the Accelerator Pump Circuit On the Bench
Save yourself a lot of aggravation by testing your accelerator pump circuit on the bench before mounting back on the engine.
Fill the flow bowl with a fluid. We use mineral spirits.Later Mercarbs fill the pump well via a slot on the side of the pump well. Some Mercarbs used an inlet check ball system. The check ball would be on the bottom of the well with a screen and retainer. On these early types, gas fills from the hole the check ball sits in. The upward suction of the pump and the weight of the fuel lifts the check ball up and fills the well.
With the venturi off, push down on the accelerator pump. Fluid should come out of the main discharge. If not, then you have a plugged passage.
Using a brass drift punch, hold the check ball in the main discharge down and push down on the pump. You should feel some resistance. If not, then seat the check ball by tapping it a few times with the drift punch. Not hard, or you will lodge the check ball.
The check ball in the main discharge is used to block off the fuel when not accelerating, otherwise fuel will siphon into the intake. When you press on the throttle, the fuel forces the check ball up and pushes the gas out.
Install the venturi with the gasket. Press down on the accelerator pump again. You should see 2 good squirts coming out of the from of the venturi. Each one going into one of the carburetor bores.