Carburetor Accelerator Pump Cam
What Is a Carburetor Accelerator Pump Cam?
When you snap the throttle open on a carbureted engine, physics works against you for a split second. Fuel is heavier than air, and as the throttle plates swing open, the sudden drop in manifold vacuum creates a brief lean condition before the main metering circuit can compensate. The accelerator pump exists specifically to bridge that gap — injecting a precisely timed squirt of fuel directly into the carburetor throat to keep combustion smooth and power delivery crisp.
The accelerator pump cam is the small but critical plastic component that governs exactly how, when, and how much fuel that pump delivers. Mounted on the throttle shaft, the cam's profile physically actuates the pump lever as the throttle opens. Its shape — the height of its lobe and the steepness of its ramp — determines the character of the entire fuel shot.
How Pump Cam Profile Affects Performance
Not all pump cams are created equal, and that's by design. Two key variables are built into every cam's geometry. First, cam lift directly affects accelerator pump stroke length and therefore the total volume of fuel delivered. A taller cam lobe pushes the pump diaphragm further, producing a larger fuel shot. Second, the ramp profile — the shape of the cam's rising edge — controls the timing and duration of that shot. A steeply rising ramp delivers a fast, aggressive blast of fuel right off idle, while a gently sloping ramp spreads the pump action over a wider throttle range for a smoother, more progressive enrichment.
Choosing the right cam is a balancing act. An off-idle stumble or hesitation on hard acceleration is one of the most common carburetor complaints, and swapping the pump cam is often the most effective first step in diagnosing it. However, it's worth noting that too much pump shot can cause a bog just as easily as too little — the ideal tune is the minimum fuel required to eliminate hesitation and produce clean, crisp throttle response.
When to Replace or Upgrade Your Pump Cam
Pump cams are inexpensive plastic components that can wear, crack, or simply be the wrong profile for a modified engine combination. Common reasons to shop for a replacement or upgrade include:
- Off-idle hesitation or stumble on rapid throttle input
- Engine bog under hard acceleration from a stop
- Upgrading to a larger carburetor, intake manifold, or camshaft that changes fuel delivery needs
- Rebuilding or restoring a carburetor to like-new performance
- Fine-tuning a race or performance build for a specific track or street application
Top Brands for Carburetor Accelerator Pump Cams
When it comes to pump cam selection, Holley is the definitive name in the category. As the manufacturer of some of the most widely used performance carburetors ever made — including the iconic 2300, 4150, and 4160 series — Holley offers a comprehensive range of color-coded pump cams, each with a distinct profile engineered to deliver different fuel shot characteristics. Their cam assortment kits make it easy to experiment and find the ideal tune for any engine combination.