On mine it was just an allen key head, use the long t handle type with a ball key head. This way it can be slanted a bit for the lower ones.
I would not bottom out the screw unless you did a rebuild of the carbs.
I start with top stb and work top to bottom sets, this is from the book:
This adjustment is NOT a periodic maintenance item and should not be touched unless all other attempts to resolve an idle speed operation problem have failed. The ignition and fuel system components must all be in good condition and operating properly. The carburetor linkage must be properly adjusted before attempting this procedure, for details please refer to Timing and Synchronization in the Maintenance and Tune-Up section.
1.With the engine top cover removed for access (and, on most models, the air intake silencer as well), make a matchmark between the carburetor body and idle mixture screw.
2.Start the engine and allow it to idle until normal operating temperature is reached.
3.Once warmed, shift the engine in forward and run at idle speed for 3 minutes.
4.If the adjustment is too lean, the engine will sneeze and backfire; to correct this, note the reference mark, then rotate the idle mixture needle (low speed screw) 1/8 turn counterclockwise. Wait 15 seconds for the engine speed to stabilize before turning the screw again. Turn the screw until you reach the highest steady engine speed.
5.If the adjustment is too rich, the engine will be rough an unsteady; to correct this, note the reference mark, then rotate the idle mixture needle (low speed screw) 1/8 turn clockwise. Wait 15 seconds for the engine speed to stabilize before turning the screw again. Turn the screw until you reach the highest steady engine speed.
6.Allow the engine to run at idle in gear for 3 minutes, then move to the next mixture screw. Repeat until all carburetor body screws have been adjusted.
7.Run the engine at or near Wide Open Throttle (WOT) for 3 minutes and then reduce speed to idle, leaving the motor in gear. The motor should not stumble, spit or backfire. If any of these problems are found, repeat the adjustment procedure, making small adjustments only, until the engine operates normally.
