¶ When you open the throttle the vacuum in the Carburetor throat goes away. As the engine speeds up the velocity of the air through the Venturi of the carburetor goes up sucking gas out of the Needle Jet and Main Jet. There is a momentary lag in air speed that can result in a lean condition. If you add more fuel during this speed lag, it prevents this lean condition. The Accelerator Pump pumps in this needed extra fuel.
The Accelerator Pump is activated by the Throttle Linkage. This linkage pushes the Accelerator Pump Diaphragm through a rod connected to the Diaphragm. A spring pushes the Diaphragm. This causes a chamber below the Diaphragm to fill with gas, from the float bowl, through a one-way valve (first). When the throttle is opened, the throttle linkage pushes the Diaphragm rod. The rod pushes the Diaphragm against the spring forcing gas through another one-way (Second) and through the Accelerator Pump Orifice. Now that Orifice looks to me like you can change it out to a different size if you could get a different size, which you can't!
Sometimes Accelerator Pumps pump to Multiple Carburetors.
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