Carb Class: Air Bleed Basics
You've heard of them, but what do they do? Air bleeds, sometimes referred to as "air jets" or "air bleeders" play a vital role in the operation of your carburetor. Air bleeds are responsible for determining the amount of air that will mix with each circuit in the metering block. Virtually every carburetor you come across will have these which make this a universal discussion. The amount of air bleeds a carburetor will have is dependent on the number of throttle bores and circuits the carburetor has. In the context of racing carburetors, it helps to think of them as (4) one barrels. Most racing carburetors will have either 8 or 12 air bleeds depending on whether they are 2 or 3 circuit. 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 racers have complained of poor idle quality and no response to adjustment of their mixture screws, this is due an incorrect idle bleed. Many idle issues can be addressed by simply adding or taking away air.
Intermediate Bleed: The intermediate bleed is found on 3 circuit carburetors ONLY. The intermediate bleed provides air for the 3rd circuit. The intermediate circuit is only adjustable externally by the air bleed and to tune it otherwise would require you to take the fuel bowl and metering block off of the carburetor. On most large flange carburetors this would be the bleed found in the middle.
High Speed Bleed: The high speed air bleed or also referred to as the "main bleed" correlates to the main system. The high speed air bleed controls how much air is fed to the emulsion channels of the metering block. To explain further, the emulsion channels distribute that air further to different parts of the main well where it mixes with fuel and ultimately goes to the booster. The high speed air bleed is generally located closest to the squirter when looking at most race carburetors.
Now that we know what each air bleed does, how do we tune them? Tuning with air bleeds is often easier than anything else on a carburetor. Most modern race-style carburetors have screw-in air bleeds that can be exchanged externally with a simple hand tool. Gone are the days of pin drills and dial calipers. 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 = more fuel = richer idle), this will help you get your mixtures screws within the 1-2 turn range. What if you want to lean out the intermediate so you can get your car to come off of the transbrake a little 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 specs so you can always come back to where you started.