They're good carburetors. I use a small flat blade screw driver that fits in the slot to adjust the disc. It's very sensitive so you don't need to move it very far at all to make a change in the low speed adjustment which is usually not very far from center.

From the manual:

PERRY CARBURETOR: Idle adjustment by means of the idle disc.

The idle disc is equipped with two O-rings which may stick in a new carburetor. Before you attempt adjusting the carburetor, let a generous amount of fuel run into the carburetor; then use a screw driver to turn the idle disc to the right and left to permit some oil to cover the O-rings. Reposition the slot of the disc on the center mark. Proceed now to adjust the carburetor for full throttle. Open the barrel completely, and adjust the mixture by turning the needle valve. Turning it clockwise leans out the mixture, turning it counterclockwise enrichens it. To insure that the engine runs properly when the model climbs vertically, you must hold it vertically with the engine running. If needed, you slightly open the needle valve until the engine runs smoothly in this position.

After this is done, you adjust the "idle speed" by setting the screw which is located by the air intake. The carb barrel stop screw on the top.....

The idle mixture is adjusted by turning the idle disc with a screw driver. If you turn the disc towards (-) the mixture becomes leaner, and in the opposite direction towards (+) it becomes richer.

"IF THE ENGINE SPEEDS UP WHEN IDLING, AND THEN STOPS, THE MIXTURE IS TOO LEAN. IF THE ENGINE SLOWS DOWN AND STOPS, THE MIXTURE IS TOO RICH. THE ADJUSTMENT IS VERY SENSITIVE. IN MOST CASES, MOVING THE DISC A COUPLE OF THOUSANDS OF AN INCH SUFFICES."

If the carburetor is properly adjusted, the engine runs smoothly even at the lowest idling speed. Also, it must pick up speed even after lengthy idling smoothly without bucking.