1) The "blue" colored screw in the image is the
duration adjustment screw. Underneath the head
of the screw is a lock nut and the bracket arm that
the screw is threaded into.
To decrease the amount of fuel squirted  the adjustment screw is turned "clockwise" or in. 
To increase the amount of fuel squirted the adjustment screw is turned "counter clockwise" 
or out.
To get the max, amount of decrease remove the
screw and take the lock nut off of the screw.
Then replace the screw into the cam arm, then
replace the lock nut underneath the arm on the 
screw. this gives you another 3/32" of an inch
in which the screw can be screwed in.
 

2) Turn the adjustment screw fully clockwise or IN.
Start your engine, Blip your throttle open,
if the engine stutters, (hesitates), it is not getting enough fuel
so turn the screw counter clockwise to increase the
amount of fuel squirted from the accelerator pump
into the carburetor.
Keep blipping the throttle and adjusting the screw
till the carburetor starts to cough. Stop here and
turn the screw back in till the coughing stops.
Tighten down the lock nut.
This should give you the best throttle response with
the least amount of coughing and backfiring out the 
carburetor.
 
 

This is the "Green" colored screw just above the
Blue duration adjustment screw.
The" Mucker" said it best,,,,,
This upper screw adjusts the "timing" of the 
squirt. By altering it's setting, you can advance or delay the onset of the fuel squirt. But probably, it won't have to be touched.