The procedure is as follows for a walbro carby.
Step 1: Bottom both screws.
Step 2: Back off the 'H' (high) screw one turn.
Step 3: Back off the 'L' (low) screw one and a quarter turns.
Step 4: Start machine then open 'L' screw till you do not have any hesitation (or very little) going up to full RPM'S.
Step 5: On full thrittle adjust till the machine is running at maximum RPM'S and then back off the 'H' screw by 1/8 of a turn.