Choosing between a 30cc and 50cc accelerator pump for a Holley 4165 (a spread-bore double pumper) usually comes down to the "off-the-line" demand of your engine. In the world of carburetors, more isn't always better—it's about matching the fuel shot to the engine's gulp of air.
The 30cc Accelerator Pump
This is the standard for the vast majority of street applications.
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Best for: Light-to-medium weight vehicles with "normal" stall converters and street gears.
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Why choose it: It provides a crisp, sufficient shot of fuel to cover the transition from the idle circuit to the main boosters without over-riching the mixture.
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Pros: Better fuel economy and cleaner spark plugs during city driving.
The 50cc Accelerator Pump
This is a high-volume pump designed for specific "problem" scenarios.
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Best for: Heavy vehicles, engines with large intake manifolds (like tunnel rams), or cars with very low-speed "stumble" issues.
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Why choose it: If you have a massive plenum or a high-duration cam that causes a significant "lean bog" when you floor it, you need the extra volume and duration of the 50cc shot to keep the engine from stalling.
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Cons: It requires a different pump cam and often a larger discharge nozzle (shooter). If you use this on a mild street engine, it will likely "slug" the engine with too much fuel, causing a slow, lazy response.
Comparison at a Glance
How to Decide
If you are currently experiencing a stumble or hesitation right as you hit the gas:
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Check your "Shooters": Try a slightly larger discharge nozzle first (e.g., move from a .028 to a .031).
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Adjust the Cam: Try a more aggressive pump cam position on your 30cc pump.
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The 50cc "Nuclear Option": Only move to the 50cc pump if you’ve exhausted nozzle and cam tuning and the car still leans out under heavy acceleration.
Since the 4165 is a mechanical secondary carb, it’s much more sensitive to pump tuning than a vacuum secondary model.
