2006 SD Johnson 4 Stroke 9.9-15HP Outboards Service Manual, Page 111Get this manual
FUEL SYSTEM CARBURETOR OPERATION
Accelerator pump
Only 15 HP models are equipped with an accelerator pump systemWhen quickly accelerated, the engine draws large volume of air with fuel lagging behind causing the mixture to be transitionally leanThe accelerator pump compensates for such fuel deficiencyThis is plunger type pump that pressure feeds fuel, at the time of quick acceleration, to the acceleration pump nozzle, which sprays fuel into the main boreWith this provision, the engine can be accelerated smoothly
Operation of acceleration pump
When the throttle lever is opened quickly, the accelerator pump lever moves in the direction indicated by the arrow with spring force causing the plunger to be depressedBecause the lower check valve closes due to pressure, fuel inside the accelerator pump chamber forces the upper check valve open, causing fuel to be sprayed to the main bore through the acceleration nozzleWhen the throttle lever returns, the accelerator pump lever moves in the opposite direction of the arrow, which causes the plunger to be returned by the force of lower spring At this time, the upper check valve is closed by spring force and the lower check valve opens, allowing fuel to enter the accelerator pump chamber123456789Accelerator pump lever Plunger Acceleration nozzle Lower check valve Upper check valve Spring Spring Throttle lever Lower spring