Honda BF9.9A-BF15A Outboard Motors Shop Manual., Page 122Get this manual
TWO-SHAFT BALANCER SYSTEM OUTLINE
The Honda Outboard Motor BF9.9A115A have 2-cylinder, 4-stroke engines2-cylinder, 4-stroke engines generally have cranking angle of 360, because of their constant interval ignitionWithout balancer, there is primary inertia forceWith two-shaft balancer offset at the top of the cylinder barrel, the primary inertia force reciprocal can be counteracted, BALANCER The vibration source of the engine is reciprocal inertia force generated when the connecting rods go up and down
I FORCE T.D.C
4 crank weight is generally mounted reduce the inertia forceMounting the crank weight as shown allows the weight's centrifugal force reduce the inertia force of the rodsHowever, crank weights produce their own horizontal vibrationThe balancer system counteracts inertia force as well as the crank weight's horizontal vibration