That is the mixture screw as stated previously, but just turning it won't necessarily improve your idle. This mixture screw is to improve the fuel to air ratio at lower speeds. It will not adjust your idle speed correctly.
The idle screw is actually closer to the flywheel, just under the timing plate linkage:
Adjust this first. Once the idle is high enough to keep the motor running on it's own, you can begin to adjust the mixture screw. If the motor is beginning to slow down on it's own, it's running rich. Turn the screw inward in small adjustments (1/4 turn) and wait a few seconds. If the idle increases, then adjust again. If it starts sneezing/coughing, then turn it back out by 1/4 turn.
Do this for both mixture screws in the upper and lower carbs.
Good luck!