Perhaps one of the most misunderstood carburetor tuning elements are the air bleeds. Playing a vital role in the operation of a carburetor, air bleeds are responsible for determining the amount of air that will mix with each circuit in the metering block. Virtually every carburetor has air bleeds, however they are not always changeable, as they are in QFT's carburetors.
The number of air bleeds a carburetor will have is dependent on the number of throttle bores and circuits the carburetor has. Most racing-style carburetors will have either 8 or 12 air bleeds, depending on whether they are two- or three-circuit as each barrel will have one bleed per circuit.
Idle Air Bleed: The idle air bleed could be the hardest working one of them all. Air to be mixed with idle fuel is provided by the idle air bleed. The idle mixture screws rely on air provided by this bleed. Often engines with poor idle quality and no response to adjustment of the mixture screws have incorrect idle bleeds. Many idle issues can be addressed by simply adding or taking air away from the idle system.
Intermediate Bleed: The intermediate bleed is found on three-circuit carburetors; it provides air for the third, or intermediate circuit, which is only adjustable externally by the air bleed. On most large-flange carburetors this would be the bleed found in the middle. To lean out the intermediate system so the engine will come off idle cleaner, increasing the size of the intermediate bleed will lean out that circuit. Just remember that too much of a good thing can get you into trouble. Before fine-tuning a carburetor you should take note of the stock spec so you can always come back to where you started.
High-Speed Bleed: The high-speed air bleed, also referred to as the main bleed, is part of the main system. The high-speed air bleed control how much air is fed to the emulsion channels of the metering block. The high-speed air bleed is generally located closest to the accelerator pump squirter.
Tuning with air bleeds is often easier than anything else on a carburetor. Air bleeds often resemble a main jet only slightly smaller. Tuning is simple in that you just need to remember the size of the hole determines how much air is coming into the carburetor. If you want to richen up the idle, simply replace your idle air bleed with a smaller one (less air in the system equals more fuel and a richer idle), this will help get the mixtures screws within the one-two turn range.
High-speed bleeds not only affect the mixture, they also impact the fuel curve. As an example, if the mixture gets richer at high rpm, a larger bleed will help.
While electronic fuel injection is high tech, carburetors remain a viable alternative to complicated and expensive electronic fuel-injection systems. Carburetors like those from QFT are reliable and infinitely adjustable, and they're affordable too. What else can you ask for?
1. John Beck at Vintage Hot Rod/Pro Machine freshened our 389 Pontiac then bolted on a new Quick Fuel Technology 750 SS carburetor. These carbs not only look great but they work that way as well.
2. One of the most basic, and often overlooked, carburetor adjustments is float height. On QFT carbs the adjustment is external.
3. Floats are adjusted by loosening the lock screw and then turning the adjustment nut clockwise to lower the float, counterclockwise to raise it.
4. A sight glass in each bowl makes setting the floats easy and it's much safer than having to remove a plug to see the fuel level and running the risk of spilling gas on a hot engine.
5. Vacuum secondary carburetors will have idle mixture screws on the primary side (1), mechanical secondary carbs often have them on the secondary side as well (2). The spring-loaded screw (3) is used to hold the primary throttle valves slightly open for idling.
6. This QFT 750-cfm carburetor has an adjustment screw to hold the secondary throttle valves open to add additional air and fuel at idle.
7. QFT uses a unique vacuum-operated secondary system. On carbs so equipped, adjustments to opening timing and rate are made with an adjustment screw rather than disassembling the unit and changing springs.
8. On mechanical carbs changing links alters the secondary opening rate. Options are: black link, secondary opens when primaries are 60 percent open; gold link, 40 percent; silver link, 1 to 1.
9. Accelerator pumps are operated by cams operating on actuating levers (vacuum secondary carbs have one pump, mechanicals have two). Various cam profiles are available. QFT advises that engines that have more load generally require more initial pump shot while combinations with less load (e.g. high stall torque converters) can benefit from less pump shot.
10. For quick starts and to reduce stumbling during warm up, an electric choke is used. One 12V supply wire is all that is required.
11. Quick Fuel metering blocks are machined from billet aluminum for precise fuel metering. Note the two changeable jets and the power valve between them.
12. This is the side of the power valve exposed to vacuum. For tuning purposes valves that open at various vacuum levels are available.
13. The jet-like orifices adjacent to each venturi are air bleeds. This carburetor allows you calibration of the idle and high-speed systems for more performance and or fuel economy.
14. Quick Fuel's newest offering is the Black Diamond Series. These new carbs introduce QFT's high-temperature coating technology for select performance and street carburetors.