Holley Carburetor Air Bleeds [Archive] - Chevy Nova Forum

: Holley Carburetor Air Bleeds


1966Nova
19th-May-2011, 12:11 AM
I have a Holley 4150 Street HP 750 which has the adjustable air bleeds. The question that I have can anyone give me any and all information on the air bleeds. If they are adjustable how do you know what size to use, etc. Any help on this would be appreciated. :confused:

levisnteeshirt
19th-May-2011, 11:28 AM
what do you have it on ?? i've not messed with them a whole lot myself ,, from what i read on 'em ,,, they're used to get a big carb to work where maybe you think it shouldn't

if you read the old holley books ,, they didn't think much about adjustable air bleeds ,, they just recommended matching the right size carb for the job , not air bleeds

The Big Al
19th-May-2011, 11:33 AM
I guess if your hand fit in a blender you need to stick your hand in it!!!!!:rolleyes:

Because you ask!


AIR BLEEDS:
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.

ZNova
19th-May-2011, 03:08 PM
Here are a couple of excellent articles from Car Craft on tuning with air bleeds. The second article details some tuning efforts at the track with a 434 SBC in a 55 Chevy, dropping the et by over 0.5 second.


http://www.carcraft.com/techarticles/tuning_with_air_bleeds/index.html

http://www.carcraft.com/techarticles/ccrp_0607_engine_tuning_guide/index.html

stock z/28
19th-May-2011, 05:08 PM
Some of the old 780 (and I think the 850) that were factory on engines like the 375-425HP 396-427s and 302s, and some 350s had adjustable idle bleeds and were fairy easy to kind of tune. The issue (in my opion) is evaluating what you have gained or lost with outthe abilty to accuratly measure the a/f ratio at some degree of partial and/or full load. I guess if you had an accurate wide band system it might help.

I have added the Chevrolet style adjustable bleed system to some standard (4781-4780-47779) Holley's useing a modified set screw, and it worked great on automatic cars with "serious" cams and tighter converters.

1966Nova
19th-May-2011, 06:28 PM
ZNOVA thanks for the articles and big Al thanks for the information. I had already found that on the internet but thank you anyway. I have had firing and carburetor problems since I bought the car. I have got the firing problem solved but haven't figured out the carburetor problem yet. Last Friday a friend and myself got the firing fixed and got the car started for the first time in two months. I knew the plugs were not very good so the next morning I went to the parts store and bought new plugs and put them in. The car had not run that good since I have had it. I drove the car around the block and was very pleased. On three different occasions I drove the car since I put the new plugs in it for a total of ten to fifteen miles (can't afford to put plugs in it every 10 to 15miles). The last time I drove it on the way home it started poppin back through mufflers. Got home and the plugs are as black as black can get, also when coming home the last time I stood on it a couple of time to try and clear it out but it didn't, behind me was the worst cloud of black smoke. Being said a person told me since it had removable air bleeds that I needed to find out what size air bleeds that originally came in the carburetor from Holley and to see if they had been changed our altered. Knowing what I have found wrong with the car after getting it home there is no telling what the guy that I bought the car from has done to it.

64blue
19th-May-2011, 06:49 PM
The outside ones are for idle - the inside pair for speed. Basically, if rich at idle you can go up in size, like from 70 (fairly std) to 73 - 74 to add more air to compensate for a fat / rich idle condition. If all else fails you can reduce the IFRs on the metering block to reduce fuel flow amount, thereby leaning out a
rich condition. If 4 corner idle system you can turn out the rear mixture screws
by 1/2 turn only to further reduce richness. Regardless of what you read about all the screws being turned out the same, it ain't true, it really applies to the specific application, like a cammed motor that has low vacuum.

levisnteeshirt
19th-May-2011, 07:21 PM
Here are a couple of excellent articles from Car Craft on tuning with air bleeds. The second article details some tuning efforts at the track with a 434 SBC in a 55 Chevy, dropping the et by over 0.5 second.


http://www.carcraft.com/techarticles/tuning_with_air_bleeds/index.html

http://www.carcraft.com/techarticles/ccrp_0607_engine_tuning_guide/index.html

good article ,,, thats the ultra HP carb there i think ,, if ya have a carb thats so adjustable , you gotta have the tools like he has to get them dialed in IMO

1966Nova
19th-May-2011, 07:51 PM
levisnteeshirt are you asking what motor is it on. If that is what you meant it is a 383 stroker. :yes:

DERYL ATANASU
19th-May-2011, 09:17 PM
Possibly weak ign. coil. Or not getting 12 volts. What jets are in the carb. now? More info. on your combo might help.

1966Nova
20th-May-2011, 11:50 AM
Thanks for the information but what I need to know is WHAT SIZE AIR BLEEDS CAME IN THIS CARBURETOR WHEN IT WAS NEW?????? I know very little about the engine except that I was told this:
Scat forged 383 stroker kit w/ 10 to 1 comp (forged rods & pistons)
Comp Cams Gold roller rocker assembly
World Sportsman II cast iron heads
Eldebrock performer alum. intake
Holley HP Street 750 carb
Also the jets that are in it now are 72 primary 76 secondary.
Anyone out there know the answer on the AIR BLEEDS?