Carburetor Accelerator Pump

What Is a Carburetor Accelerator Pump?

If you've ever experienced a stumble, bog, or brief hesitation when you nail the throttle on a carbureted engine, the accelerator pump is likely at the heart of the issue. Carburetor accelerator pumps are small but essential components that solve a fundamental physics problem: when the throttle snaps open, air rushes into the engine almost instantly, but fuel — being heavier — lags behind, momentarily leaning out the mixture.

Carburetor accelerator pumps address this directly. As the Wikipedia entry on carburetors explains, "while the main metering circuit can adequately supply fuel to the engine in steady-state conditions, the inertia of fuel (being higher than that of air) causes a temporary shortfall as the throttle is opened" — which is precisely why every carburetor includes one. When you press the accelerator pedal, a small piston or diaphragm pump injects extra fuel directly into the carburetor throat to bridge that gap and restore the correct air-fuel ratio.

How Does an Accelerator Pump Work?

Most accelerator pumps rely on one of two mechanical designs: a piston-type or a diaphragm-type. In a piston-style system, throttle linkage drives a plunger down into a fuel-filled bore, forcing a precisely metered squirt of gasoline through a discharge nozzle and into the carburetor's air horn. A diaphragm-based design achieves a similar result with a flexible rubber membrane that responds to linkage movement. Inlet and outlet check valves control the direction of fuel flow, ensuring the pump refills when the throttle closes and discharges only on opening.

Beyond curing acceleration stumbles, the accelerator pump also serves a secondary role: it can be used to prime a cold engine with extra fuel before a cold start, helping the engine fire up more reliably in low-temperature conditions.

Signs of a Failing Accelerator Pump

Accelerator pump components — particularly rubber diaphragms and pump cups — are subject to wear, heat cycling, and fuel degradation over time. Common symptoms of a failing pump include:

• Hesitation or bog on quick throttle application
• Sputtering or stalling during hard acceleration
• Hard starting, especially after the engine has been sitting
• Poor fuel economy caused by an improperly metered fuel charge

Fortunately, replacement is typically straightforward. Accelerator pump assemblies are available as standalone components or as part of a carburetor rebuild kit, and many include the pump cup, spring, and necessary hardware.

Performance Tuning Considerations

For performance and race applications, the accelerator pump is also a tuning variable. The volume of the pump shot, the duration of fuel delivery, and the shape of the pump cam all influence how the engine responds to different throttle inputs. As a general rule, the ideal accelerator pump tune delivers the least amount of fuel needed to eliminate lean hesitation — too much fuel can cause an equally problematic rich bog.

Top Brands for Carburetor Accelerator Pumps

Whether you're restoring a classic muscle car, rebuilding a daily driver, or tuning a performance carb, these trusted brands have you covered:

• Edelbrock — One of the most recognized names in carburetion, Edelbrock offers a full lineup of carburetor accelerator pump assemblies engineered specifically for their popular Performer Series and Thunder Series AVS carburetors. Their pumps feature Viton cups, which offer superior resistance to fuel degradation and heat compared to standard rubber, making them a go-to choice for both street and performance rebuilds.
• Mevotech — Known for their rigorous quality control and broad parts coverage across the aftermarket, Mevotech brings their OEM-grade engineering standards to carburetor replacement components. Their parts are designed to meet or exceed original equipment specifications, making them a reliable choice for restoring proper fuel delivery on a wide range of applications.
• Revtek Suspension — Revtek rounds out this category with a carburetor accelerator pump option that provides a practical direct-replacement solution for specific applications, offering a straightforward path to restoring correct throttle response.