Honda BF9.9A-BF15A Outboard Motors Shop Manual., Page 123Get this manual
HONDA BF9.SA-BF15A
OPERATING PRINCIPLE OF THE TWO-SHAFT BALANCER
[WHEN CRANK ANGLE IS 90" The upward or downward inertia force on the piston is zero, but the inertia force (50) is applied horizontally on the crank weightTherefore, by applying two balancers of 25 to the 50 opposite direction, the imbalance inertia force will be zero
[4] WHEN CRANK ANGLE IS 0" The inertia force (100)applied upward or downward to the piston is counteracted by downward inertia force on the crank weightThe remaining force will be 50This will be further counteracted by the downward inertia force on the two 25 balancers, so that the entire imbalance inertia force will be zero