Honda BF20A-BF25A, BF25D-BF30D Outboard Motors Shop Manual., Page 53Get this manual
HONDA BFSOAS5A
NOTE The No3 carburetor is the synchronization base carburetorThis carburetor will not have carburetor synchronization adjuster screwAdjust the carburetors to get the least amount of vacuum difference between cylindersAs the manifold vacuum difference decreases, the idle speed will become more stableEach increment on the vacuum gauges indicates 25.4 mmin) Hg
11I ADJUSTING SCREW
9) Turn the No1 and No2 carburetor adjusting screws so that the vacuum difference between all cylinders is 20 mm (0.75
in) Hg or less10) After each adjustment, check the idle speed and adjust if necessary by turning the No3 carburetor throttle stop screw (page 3-7)1 1) Snap the throttle several times and allow the engine to return idleCheck be sure that the vacuum difference between all cylinders stays within 20 (0.75 in) Hg or lessReadjust if necessary12) Stop the engine and remove the vacuum gauge adaptersInstall the 10 mm phillips head plugs and sealing washers