JEGS Tech Specialist

Pete Mitchell

So, you’ve just installed a shiny new Holley carburetor on your engine. But when you fire it up for the first drive, something feels off. Maybe the engine stutters or hesitates when you press the gas, or it doesn’t run as smoothly as you’d expect. Sometimes, it may even struggle to start, whether the engine is hot or cold.

This is because a carburetor isn’t a “set it and forget it” type of part. Although it may look simple on the outside, it’s actually quite complex. It needs proper setup and tuning to perform at its best. Inside, there are various circuits, pathways, and mechanical components that all need to work together. Plus, external factors like temperature and altitude can affect how your carburetor functions.

In this article, we’ll guide you through tuning your Holley accelerator pump to improve throttle response and smooth out performance. You’ll also learn the difference between the 30cc and 50cc accelerator pumps and which one is right for your needs.

And if you need help picking the right Holley carburetor or other components for your vehicle, our team at JEGS is here to assist. We know carburetors inside and out, and we’re happy to answer any questions you have.

Understanding The Accelerator Pump

An accelerator pump is an important part of any carburetor because it helps deliver extra fuel when you need sudden power. Normally, a carburetor’s main system, called the metering circuit, uses the speed of air moving through the carburetor to control how much fuel enters the engine. But when you press the gas pedal quickly (like when you “floor it”), the airflow doesn’t increase fast enough for the metering circuit to keep up with the demand. This delay causes a hesitation or a momentary lack of response from the engine. In some cases, you might even hear a backfire through the intake manifold.

The accelerator pump solves this problem by providing an instant burst of fuel when you hit the gas pedal. Here’s how it works:

• Fuel Source: The accelerator pump gets fuel from the carburetor’s fuel bowl through a small check valve.
• Pump Mechanism: Inside the accelerator pump, there’s a diaphragm (a flexible rubber-like part) and a return spring. This diaphragm is connected to the throttle linkage (the mechanism that opens and closes the throttle) through levers.
• Fuel Delivery: When you press the gas pedal, the throttle opens, and the levers push the diaphragm upward. This movement forces fuel out of the pump through a small passageway, which leads to the carburetor’s main air passage, called the venturi.
• Injection of Fuel: A check valve in the system ensures that the fuel remains above the float level, ready for delivery. The fuel then travels through a calibrated nozzle, which sprays it directly into the venturi, giving the engine the extra fuel it needs right away.

This quick squirt of fuel makes up for the time it takes the carburetor’s main metering system to respond to the sudden increase in airflow.

The amount of fuel the accelerator pump delivers depends on how much you open the throttle. For small, gradual throttle changes, the accelerator pump barely works because the carburetor’s main system can handle those needs. But when you make a sudden change, like stepping on the gas hard, the accelerator pump kicks in to prevent engine hesitation.

Holley Carburetors: In Holley carburetors, all models have an accelerator pump on the primary side (which controls the first set of throttle plates). However, only the models called Double Pumpers have an accelerator pump on the secondary side (the second set of throttle plates). Other models use a vacuum to open the secondaries, so they don’t need an additional accelerator pump.

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30cc vs 50cc – What’s The Difference?

Holley carburetors typically come with a 30cc accelerator pump, which is more than enough for most street driving. This size delivers the right amount of fuel for smooth acceleration under normal conditions, such as everyday commuting or casual driving.

However, for high-performance engines, where there’s a lot more horsepower and the engine is under heavier loads (like in racing or towing), there’s an option to upgrade to a 50cc accelerator pump. This larger pump delivers more fuel for longer periods, helping the engine respond better to sudden throttle demands.

Key Differences:

• 30cc Accelerator Pump: Perfect for street cars and light performance. It provides quick, responsive fuel delivery without being excessive.
• 50cc Accelerator Pump: Ideal for high-horsepower engines or extreme conditions where more fuel is needed. This upgrade is generally unnecessary for regular street driving and can be considered overkill for most non-racing applications.

Visual Difference: The 50cc pump has a larger diaphragm than the 30cc pump. This deeper, taller design allows it to hold and deliver more fuel to the engine.

Step-by-Step Adjustment Process

Adjusting the accelerator pump is key to getting the right fuel delivery when you hit the gas. The two most important parts to adjust are the nozzle (also called the squirter) and the pump cam. Here’s how you can make sure everything is set up correctly:

1. Check the Pump Linkage Clearance

Before anything else, check that the accelerator pump linkage is set up properly. Holley recommends a clearance of 0.015 inches between the lever arm and the pump linkage when the throttle is fully open (not when it’s at rest). To do this:

• Have someone press the gas pedal all the way down (with the engine off).
• While they hold the throttle wide open, measure the clearance between the lever arm and pump linkage.

This ensures that the pump will deliver fuel properly at wide-open throttle.

2. Tuning the Nozzle Size

The size of the nozzle (or squirter) determines how much fuel is injected into the engine when the accelerator pump is activated. Here are the basic guidelines:

• Stock sizes: The standard nozzle size for the primary circuit is 0.031 inches, while the secondary circuit uses a 0.028-inch nozzle.
• Upgrading the nozzle: If you need more fuel for faster throttle response, you can increase the nozzle size. However, it’s important to increase both the primary and secondary nozzles to keep the fuel delivery balanced. This helps avoid putting too much strain on the primary circuit alone.

3. Adjusting the Pump Cam

The pump cam controls how quickly the fuel is delivered by the accelerator pump. Holley carburetors offer two mounting positions for the pump cam:

• Position 1: This is the default setting, designed for engines idling below 900 RPM. It delivers the fuel sooner for better response at lower speeds.
• Position 2: This setting delays when the fuel is delivered, which is better for engines idling between 900 and 1,000 RPM. It allows for a smoother fuel curve at slightly higher idle speeds.

These adjustments help you fine-tune the accelerator pump for the specific needs of your engine, whether you're cruising on the street or racing at the track.

Fine-Tuning For Optimal Performance

When it comes to fine-tuning the accelerator pump, the goal is simple: use the least amount of fuel necessary to get the best acceleration. This ensures you're getting the most efficient performance out of your engine without wasting fuel.

Holley recommends this tuning philosophy because it strikes the right balance between power and fuel economy. By delivering just the right amount of fuel when you press the gas, you ensure the engine responds quickly and smoothly without overloading it with excess fuel.

To achieve this:

• Start with small adjustments and test how the car accelerates.
• If the engine hesitates or feels sluggish when you press the gas, it may need more fuel.
• If acceleration is smooth but you notice fuel consumption is higher than it should be, try reducing the amount of fuel delivered by the pump.

The key is to find that sweet spot where the engine responds instantly, but you’re using only the fuel needed for quick and efficient acceleration.

JEGS – Your Trusted Partner For Tuning

Looking to get the most out of your engine? At JEGS, we’re here to help you unlock peak performance. Whether you need advice on choosing the right aftermarket parts or guidance on setting them up for maximum power, our team has the experience to assist you every step of the way. With decades of knowledge, we’re eager to share our expertise to ensure your engine performs at its best.

Every product you purchase from JEGS comes with full warranties and is supported by our excellent after-sales service. That’s why thousands of gearheads nationwide trust us for their performance needs.