Honda BF8, BF9.9 and BF10 Outboard Motors Shop Manual., Page 289Get this manual
17OPERATION ACCELERATOR PUMP2Starting Enrichment (SE) SYSTEM
1ACCELERATOR PUMP
When the throttle valve opens quickly for acceleration, etc, the air flow velocity at the venturi slows down quickly, too, decreasing the vacuumBecause the induced fuel amount decreases, compared to the suction air amount, the airlfuel mixture sucked into the cylinder temporarily becomes too lean and the engine can flame outThe acceleration pump is equipped to prevent the lean aidfuel mixtureIn short, it is designed to increase the fuel amount and make the airlfuel mixture richer when the throttle opens quicklyWhen the throttle valve opens, the acceleration pump piston is pushed down via the pump rodBecause the inlet check valve is closed this time, the pump chamber is pressured and the outlet check valve is openedTherefore, the fuel is injected from the nozzle into the main boreWhen the throttle valve is closed, the acceleration pump piston is pushed back by the spring force, the inlet check valve opens, and the fuel in the float chamber enters the pump chamberThe outlet check valve is closed this time, preventing suction of the air through the nozzle