Honda BF20A-BF25A, BF25D-BF30D Outboard Motors Shop Manual., Page 270Get this manual
8) The No.3 carburetor is the synchronization base carburetorTurn the No.1 and No.2 adjusting screws so that the vacuum difference between each cylinder is 20 mmHg (0.75 inHg) or lessSnap the throttle several times and allow the engine to return to idleCheck to be sure that the vacuum difference between each cylinder stays within 20 mmHg (0.75 inHg) or lessReadjust if necessary9) Stop the engine and remove the two tube clips [I]No.1 CARBURETOR ADJUSTING SCREW
10) Remove the vacuum gauge adapters from the intake manifold, and install the 10 mm plugs and mm seating washers11) After adjustment, perform idle speed inspection and adjustment (P3-2)