Carburetor FAQ

  1. Mrmopartech
    There always seems to be a lot of questions relating to carburetion, especially questions such as which carburetor to use or how much cfm the engine should have. The problem with answering these questions is that the answers vary with engine size and tuning rpm. A related problem is that all carburetor manufacturers don't use the same rating system for their carburetors, and some don't rate their carburetors' "cfm" at all. Cfm stands for cubic feet per minute, which is used for rating the air flow of a carburetor. OE manufacturers (such as DaimlerChrysler) don't test carburetors for air flow. Since the advent of emissions laws, OE engineers test carburetors for fuel flow because it relates to the richness level in the engine which relates to emissions.

    Air flow relates better to horsepower, high rpm performance, and inversely, driveability. To use it, first you have to obtain air flow data. The only good source is Holley, but don't try to compare 2-Bbl. to 4-Bbl. data. So, narrowing our question down to 4-Bbl's. and V-8s, we can come up with the following general recommendations. We'll select an engine tuning rpm of 6,500 rpm (fairly average performance number).

    If you want to tune for 5,500 rpm, then subtract 100 cfm. If you want to tune for 7,500 rpm, then add 100 cfm. Keep in mind that these are only approximates. You're not going to find 755 and 840 cfm carburetors. You'll find 750 and 850 cfm carburetors. Also keep in mind that tuning for 6,500 rpm is going to make these engines have somewhat poor driveability. The 5,500-6,000 rpm numbers would be better for dual purpose engines. Additionally, tuning for 6,500 or 7,500 doesn't necessarily mean that the resulting engine package would perform at this rpm. Many more pieces than just the carburetor are required to run high rpm.

    Check and record the accelerator pump shooter size. Also check the accelerator pump and adjust it for maximum travel. Rod adjustment to the hole closest to the pivot is recommended as the initial starting point. Be sure the metering rods move up and down smoothly by pressing lightly in the center of the metering rod bar.

    The point at which the throttle opening opens the air door is determined by the number of turns on the spring. To adjust the spring, two screwdrivers (or a special tool) are required. The adjustment is on the driver's side. The outer slotted ring is the locking screw and must be loosened before the spring can be adjusted. Once loosened, the air door will hang vertically. Adjust the inner screws (which tensions the spring) until it just closes the air door. Then turn the screw and tension the spring an additional 1-1/2 to 2 turns. Once set, always use the same technique for future adjustments. The fewer turns, the quicker the air door opens-typically used on faster cars. The more turns, the slower the door opens-typically used with automatics and street tires.

    The amount that the air door opens can be adjusted at the track without disassembling the carburetor. This adjustment can change the amount of air flow that goes through the carburetor and also change the fuel-air ratio (richness level) of the mixture delivered to the engine. Before you make any adjustments you must know how to accurately measure the amount of opening.

    The service manual recommends that the air door measurement be made at the top of the air door blade with the door wide open. We find that it's more accurate to measure from the tip of the rear edge of the air door to the back of the plastic part of the carburetor housing in the secondary throttle bore with the blade at the wide-open position. See Chart.

    Changing the way we measure the air door opening will change the setting specification. The standard service manual specification is .500". We use .750" to .900" for most applications.

    Remember, the first thing to do is to measure your specific carburetor and record its reading. Once the car has been baselined at the track at this setting, adjustments can be made.

    Adjustments to the air door opening are made by bending a tang or tab on the left side of the air door at the top. Use pliers to bend. Double check that the air door doesn't hit the choke rod at its new opening. Always measure the opening using exactly the same technique. Adjust as follows: open up to increase power and lean out the mixture; close down to enrich. If it develops a miss in high gear at high speed, you have opened it up too far and it's too lean. Close it back down for best performance.

    If all else fails, keep in mind the golden rule of carburetor selection-don't over carburete!

    Engine size: CFM@6500

    318 625
    340&360 655
    383 725
    400&426 755&805
    440 840
    500 945

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