The Carburetor Shop LLC

204 East 15th Street

Eldon, Missouri 65026


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When choosing a carburetor for your car, a little homework will often pay large dividends. Consider the following questions:

  1. Is your car a 'numbers matching show car'?

If yes, you are generally restricted to the original carburetor or a duplicate thereof.

2. If no, is your car going to be used for sanctioned racing?

If yes, check with the sanctioning body for their regulations.

  1. If no, is your car going to be based on some factory engine (Example. You have a 1966 Pontiac catalina with a regular fuel 389 and two barrel carburetor. You want more horsepower. Consider upgrading THE ENTIRE ENGINE to 1966 Pontiac catalina premium fuel 4 barrel carburetor specifications, or further to GTO specifications.) By duplicating a factory engine, the engineering has been done for you by the factory. YOU KNOW THE THING WILL WORK!
  1. Assuming you answered no to all of the above, you accept responsibility for doing your own engineering, and the following set of questions are designed to help you make your choice. Please remember that any choice you make will be a compromise. You may choose to maximize horsepower, drivability, fuel economy, changability, appearance or initial cost. Maximizing one category will result in reductions in one or more of the other categories.
  1. Is it legal? The Federal Clean Air Act of 1966 took effect with the 1968 model cars. The Federal Clean Air Act applies to all vehicles licensed in all 50 of the United States. Additionally, some states may have emission requirements more stringent than the Federal level, but the Federal level is the minimum in all 50 states. (1998 addition). It is being reported in many publications that California vehicles through the year 1974 are now exempt from smog emission due to California S.B. 42. THIS IS INCORRECT! Remember government 101 (the state cannot override the federal government). SB42 reduced California's more stringent requirements on 1968 to 1974 vehicles back to the Federal level, and removed emissions on 1966 and 1967 California vehicles.

B. Do you want to use a single carburetor or some form of multiple carburetion?

If you wish to use a single carburetor, the following equation can be used for multi-cylinder 4-stroke engines: CFM = (RPM x CID) / 3456. This equation is familiar to most enthusiasts, but understanding the equation seems to be a different story. Most apply the equation for WOT (wide open throttle) to determine the maximum size of the carburetor, which is good, but only half of the story (unless you plan to drive on the street constantly at WOT). For street use, it is also important to use the equation for your normal cruising RPM, and these that CFM figure for the primary side of your 4 barrel carburetor. This will maximize the air velocity (and the primary efficiency) for your cruising RPM. Remember that too large a venturi means too small air velocity, which means a LEAN condition! We have found that, for modern 8 cylinders of 300 CID or larger, almost without exception, a spread-bore carburetor (small primary, and large variable secondary) will perform best on the street. We have a small quantity of high performance Carter spread-bore aftermarket carburetors (with electric choke). These are part number 9800 and are rated 800 CFM (200 primary plus a variable secondary). Also, consider whether the engine is a large displacement high torque, low RPM; or a smaller displacement high RPM engine; and also the breathing capability of the engine. Chevrolet found the small block to like smaller primary and larger secondary. The 1963 Corvette 327/350 used a 575 (225P/350S) CFM unit. Pontiac found the spread-bore 750 did not do as well on a 455 as an 800 (200P/600S). Chrysler used spreadbore 850 (250P/600S) on their 440 engine.

To help you pick the size carburetor for street use, consider the following Carter aftermarket carburetor sizes:

Square-bore (secondary is variable, based on engine demand)

400 CFM 200 (P), 200 (S)

500 CFM 225 (P), 275 (S)

600 CFM 250 (P), 375 (S) (this is NOT a misprint)

625 CFM 250 (P), 375 (S)

750 CFM 375 (P), 375 (S)

950 CFM 375 (P), 575 (S)

Spread-bore (secondary is variable, based on engine demand)

800 CFM 200 (P), 600 (S)

850 CFM 250 (P), 600 (S)

1000 CFM 400 (P), 600 (S)


Assembling a multiple carburetor street set-up can be a tricky situation. Unless you wish to maximize appearance only, FOR BEST RESULTS, the number of carburetors should be a factor of the number of cylinders ie 2, 4, or 8 carburetors on an 8 cylinder, 3 or 6 carburetors on a 6 cylinder. (Four cylinder engines present their own set of problems - CALL). Before all you Chrysler 6-pack and Pontiac tri-power gearheads get your hackles up, please read further! I have tri-power on my own 1964 GTO because Pontiac put it there to maximize appearance, but a properly selected 4 barrel runs better! I didn't say tri-power won't work. It just doesn't work as well as a 4 barrel. When more than one source (carburetor) feeds a cylinder, turbulence exists at the point of join. This turbulence will cause the air fuel mixture delivered to the cylinders to break down at higher RPM. Please note that when the factorys used three two barrel units, they were primarily installed on relatively low RPM undersquare engines. Please also note that Chrysler used dual quads on their race engines with multiple carburetors, as did Pontiac with their Super Duty race engines.

Once you have determined configuration, you must choose carburetors. FOR BEST RESULTS, use:

  1. original carburetors from a similar multi-carb unit
  2. aftermarket carburetors whose manufacturer offers calibration pieces
  3. carburetors with mechanical power systems (especially important if building older units such as 2 x 2 on a flat-head Ford, etc.)

Your carburetors will probably have to be re-calibrated. Having a ready source of calibration pieces is useful. Modern carburetors utilize vacuum to determine auxilliary fuel flow (power valves, metering rods, etc.) A higher lift camshaft will lower the engine vacuum and require recalibration of the carburetor's power system. Many problems on street systems are a direct result of ignoring the vacuum signal, even when the proper size and type of carburetor are used.

Sizing carburetors for multiple units can be very tricky. Remember that two barrel carbs and four barrel carbs are rated with a different scale. Also remember that carburetors too large for your engine will run lean!

Linkage can be another problem. We recommend street dual quad systems run simultaneous linkage. That is, the throttle controls both carburetors as though there were only one. We suggest using the choke and idle circuits on both carburetors. This may mean fabricating your linkage.

The Carburetor Shop is in the business of selling, restoring and/or modifying carbuetors. We have installed multiple carburetion systems on everything from 4 one barrels on a 2 litre Ford four cylinder to dual AFB Carters on a number of racing only V-8's We have devised tooling to fabricate custom linkage, fuel lines, fittings, etc. We would like to have YOUR BUSINESS.