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1,658 Posts
Discussion Starter #1
Does anyone have a simple explination as to how and what Air bleeds exactly do? I'm just trying to figure out how guys come up with the #s they run for jets. The guy I bought my car from already did all the figuring for my Proform center section so I'm just trying to figure out how he did so. Thanks guys

Premium Member
15,966 Posts
Direct from holley Carbs

WARNING! Adjustment of the air bleeds is not recommended. A competent mechanic with a complete and thorough
knowledge of carburetors, fuel systems, and engine requirements should only perform air bleed
adjustments. Failure to follow these recommendations may result in a lean fuel/air mixture causing severe
engine damage, property damage, serious injury, and/or death.
WARNING! Air bleed sizes should not be adjusted more than six (6) sizes in any one direction from the original air
bleeds, as shipped from Holley. Air bleed adjustment beyond six (6) sizes could result in a lean fuel/air
mixture causing severe engine damage, property damage, serious injury, and/or death.
Experimenting with air bleeds is not recommended and should only be attempted by an expert carb tuner. Countless hours of
testing have been performed on expensive flow stands to obtain the proper bleed size for a given calibration. It is unlikely that a
better air bleed calibration can be obtained, however the 4150 ULTRA HP Series are equipped with removable air bleeds. Here
is some basic knowledge of how air bleeds work.
The main or high-speed air bleeds affect the entire range of the main-metering system. The purpose of the main metering
system and main air bleeds is to emulsify the fuel before entering the discharge nozzle to be discharged into the air stream in the
venturi. The fuel/air mixture becomes leaner as air bleed size is increased. Decreasing the size of the main air bleeds will
decrease pressure across the main jet, which in turn will pull more fuel through the main system creating a richer fuel/air mixture.
The main or high speed air bleeds also act as an anti-siphon or siphon breaker, so fuel does not continue to discharge or dribble
into the venturi after airflow is reduced or stopped. At high speeds, the fuel/air mixture must be on the rich side to prevent
damage to the engine.
The idle system supplies fuel at idle and low speeds. The idle system requires a richer mixture than at cruise speed. Unless the
idle mixture is richer, a slow and irregular combustion will occur, known as a rough idle. Decreasing the idle air bleed size
richens the idle mixture by increasing the pressure drop in the system. Increasing idle air bleed size leans the idle mixture by
reducing the pressure drop across the idle air bleeds. The same conditions can be created by backing out the idle mixture
screws, which will increase the pressure across the idle air bleeds, pushing more fuel from the idle well creating a richer fuel/air
ratio. The idle mixture screw is the only adjustment recommended for controlling the idle fuel/air mixture richness or leanness.

Premium Member
11,072 Posts
I find it's pretty simple to fine tune the carb using them on my Demon.
When changing main jets it was always too fat at one end of the rpm range, or too lean on the other--changing the air bleeds makes it much easier to find a happy medium.
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