Carburetor Accelerator Pump Nozzle

What Is a Carburetor Accelerator Pump Nozzle?

The accelerator pump nozzle — often called a squirter or discharge nozzle — is one of the most influential tuning components on a carburetor. When you press the throttle, the sudden rush of incoming air momentarily upsets the air-fuel mixture before the main metering circuit can respond. The accelerator pump circuit bridges that gap by firing a precisely metered shot of fuel directly into the carburetor's venturi. The nozzle is the final gatekeeper that controls how that fuel is delivered.

Here's how it works: as the throttle opens, linkage depresses the pump diaphragm or piston, pressurizing the fuel in the pump chamber. That pressure forces fuel through the metering block and out through the discharge nozzle, which is aimed directly into the primary bore of the carburetor throat for immediate mixing with the incoming air charge. A spring-loaded check ball inside the nozzle assembly acts as a one-way valve, preventing siphoning between pump strokes and ensuring a clean, consistent shot every time the throttle is applied.

Why Nozzle Size Matters

The size of the nozzle orifice is one of the most important variables in accelerator pump tuning. Nozzle sizes are typically stamped directly on the squirter for easy identification — for example, a "31" stamp indicates a 0.031-inch hole. A larger orifice delivers fuel at a faster rate with less pressure, while a smaller orifice produces a longer-duration, higher-pressure shot. As a general rule, most street carburetors perform well with nozzle sizes ranging from 0.028 to 0.037 inch, while modified or high-performance engines may call for larger nozzles to satisfy their increased fuel demand on rapid throttle application.

It's worth noting that the nozzle size does not change the total volume of fuel delivered per stroke — that is controlled by the pump cam and diaphragm displacement. What the nozzle controls is the rate and duration of the fuel shot. A smaller nozzle lengthens the shot duration, which can be beneficial at wide-open throttle in certain applications, while a larger nozzle delivers fuel faster for a more aggressive initial response. Getting this balance right is key to eliminating the dreaded off-idle stumble or throttle bog that plagues many carbureted engines.

Key Considerations When Shopping

• Compatibility: Confirm the nozzle fits your specific carburetor model (e.g., Holley 4150, 4160, or 4500 Dominator series).
• Orifice size: Start conservatively and step up in small increments. The ideal tune uses the smallest nozzle the engine will accept cleanly.
• Material: Aluminum nozzles offer lightweight durability; look for anodized finishes for corrosion resistance.
• Included hardware: Quality nozzles come with replacement gaskets, which are critical for preventing fuel leaks and maintaining proper check valve function.
• Single vs. dual nozzle: Some four-barrel carburetors use separate primary and secondary nozzles, which can be tuned independently for refined drivability.

Top Brands for Carburetor Accelerator Pump Nozzles

Holley is the benchmark for carburetor pump nozzle selection, offering an extensive range of discharge nozzle sizes designed to fit their legendary 4150, 4160, and 4500 carburetor families. With both straight-type and tube-type configurations available across a wide range of orifice sizes, Holley gives tuners the flexibility to dial in exactly the fuel shot their application demands.

Quick Fuel Technology brings a motorsports-focused engineering approach to accelerator pump nozzles. Their squirters feature an exclusive streamlined body design intended to reduce fuel pull-over at high RPM — a common issue on race engines where airflow through the venturi is aggressive. Each nozzle is stamped for easy size identification and includes replacement gaskets, making them a favorite among performance tuners building and refining race-ready carbureted engines.