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Carburetor Buyer's Guide

For many of us nothing beats a good old’ two-barrel carburetor or four-barrel carburetor or even better, a Double Pumper carburetor. Carburetors are what we know, and they easily fit under that trick air cleaner we want to run on our project car. Relatively speaking, a carburetor is an economical choice for the old build budget too. Tuning is done via a screwdriver and small carburetor option parts, and all we need to figure out is the right carburetor for our individual needs. Figuring out the most ideal carburetor is the toughest part, but we’re here to help you make an educated decision.

Can A Bigger Carburetor Increase Horsepower?

Yes, a bigger carburetor can increase horsepower, but this is a case of bigger is not always better. If you run a 600-cfm carburetor, it doesn’t mean a 750-cfm carburetor is going to make more power. Furthermore, people love saying they have a 750 Double Pumper carburetor, but just because that’s what their engine has, it doesn’t mean their combination is efficient. When figuring out what carburetor your engine needs, let’s look at the following equation:

1. Divide the engine’s displacement by 2

2. Multiply the engine’s maximum rpm by the result from Step 1

3. Divide the answer from Step 2 by 1,728

4. Multiply the answer from Step 3 by the engine’s Volumetric Efficiency percentage

The answer will be the cfm requirement for an engine’s displacement. If your engine was built by a reputable builder, many times they will provide the engine’s volumetric efficiency percentage. If not, a good rule of thumb is to use 70-75 percent for a garden-variety engine, 80-85 percent for a performance engine and 85-95 for a high performance race engine or stroker engine. If you perform this equation, you’ll discover few engines need even a 650 Double Pumper carburetor. Chances are that Chevy 350 4 barrel carburetor you need for your mild street build will be nothing more than a 600-cfm carburetor.

Are Carburetors Reliable (Can They Be)?

Yes, a 2 barrel carburetor or 4 barrel carburetor can be very reliable. After all, carburetors were used solely for almost 100 years of the automobile’s existence, even tri power carb applications using three carburetors were dependable. The enemies of a carburetor are the same with any induction system; those being corrosion, dirt, or any other contaminant, but also lack of use is known to be a detriment to a carburetor. Vital components within the carburetor, along with the mechanical linkage can become gummed up due to old fuel or corrosion. The best thing for a carburetor is regular use with fresh fuel, the right amount of fuel delivery, properly functioning ignition components and a clean air filter.

Square-Bore or Spread-Bore Carburetor?

A spread-bore four-barrel carburetor features two small primary throttle bores up front and two larger, secondary throttle bores at the rear. A square-bore 4 barrel carburetor has four like-sized throttle bores. Because of this arrangement, if you have a spread-bore carburetor and intake, you can’t bolt a square-bore carburetor onto said intake without using an adapter plate or swapping the whole intake manifold. A spread-bore intake manifold will have provisions for the mis-matched throttle bores so when you bolt down a square-bore four-barrel carburetor, the throttle bores won’t line up. If you have a Chevy 350 4 barrel carburetor, if it’s a Rochester Quadrajet, it’s mostly likely a spread-bore throttle bore arrangement. Also, some Carter 4 barrel carburetor applications are spread-bore, but the differences between primary and secondary bore sizes aren’t as drastic as they are with the Quadrajet.

Mechanical Vs Vacuum Secondary?

This is just a determination of how you want to open your four-barrel carburetor application’s secondary throttle blades. Do you want them to be engine vacuum-operated or via a mechanical linkage? A four-barrel carburetor acts like a two-barrel carburetor until more air and fuel are needed. A four-barrel carburetor using a mechanical secondary is often referred to as a Double Pumper carburetor. A vacuum secondary carburetor works off engine vacuum. Both a mechanical secondary carb and a vacuum secondary carburetor can be adjusted to fit the engine’s needs. A vacuum secondary arrangement has different springs to arrive at an optimum opening rate, while a mechanical secondary carb’s mechanical linkage can be adjusted to fit the engine’s needs. A mechanical secondary Double Pumper carburetor is more at home in an extreme performance environment like drag racing, while a vacuum secondary four-barrel carburetor is more designed for a street car. When we think of a tunnel ram carb or Holley dual quad carbs, we think of either a 650 Double Pumper carburetor or a 750 Double Pumper carburetor, whereas if we’re talking about a street-destined Ford 302 4 barrel carburetor, we think of a vacuum secondary application.

Electric Versus Manual Choke?

Regardless of if you’re choosing between a 650-cfm carburetor and a 750-cfm carburetor, you will need to decide between an electric or manual choke. This is easily explained, as well. If a 650-cfm carburetor has an electric choke, that means an electrical circuit opens and closes the choke when needed and not needed. If you have a 750-cfm carburetor with a manual choke, and we use these as random examples, the driver controls the choke via a cable inside the car. Many older cars have a manual choke you must pull when first starting up the car until the engine is warm. Then the choke handle is pushed back in once the engine can idle on its own. Many prefer the set it and forget it functionality of the electric choke, though just like some prefer a manual transmission and that “3rd pedal,” some will always choose the manual choke, so they have more control over how the engine responds when hot or cold.