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Old 09-28-2003, 10:29 PM
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air screw adjustment?

ok i just rebuilt my holley 750 with secondarys.. runs great.. but i need some help in setting up my air screws?? i set my floats already but im not exaclty sure how to adjust the air screws.. i'v read ways of doing it.. don't ya gota trun up the idle?? then set then or somthin? or set them til the idle is hi.. then turn the idle down?? i don't no.. u guys tell me.. its on my 350 sbc

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Old 09-28-2003, 10:36 PM
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Best way to do this is with a vacuum gauge, all you do is set them at the highest vacuum, if not you can set them by ear but you have to listen closely to the engine, when it`s at idle at it`s smoothest, then it`s close or on the money. the idea is to set them evenly, so after you have them set, it maybe a good idea to shut the engine down, and turn the mixture screw in and count the turns until it bottoms out real lightly. then set both sides the same amount of turns. there`s no need to raise the idle, when the mixture is set correctly, the idle should raise very little on it`s own.
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Old 09-28-2003, 10:39 PM
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ok i think i understand.. i do have a vacum gauge.. so your saying hook up my gauge.. rev it if i have to to get to peak vacum then set it right?
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Old 09-28-2003, 10:58 PM
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Not really, you set your idle speed to what its supposed to be. Then turn the screws counter clockwise until you reach the best vacuum. Then turn them in until your vacuum reading just starts to drop, and stop turning.


Here is another way and a little more accurate if you have a tachometer that you can use under the hood.. Start with the engine off and seat both screws gently, ( clockwise) Then back each out 4 turns. Start the engine and slowly turn each screw in ( clockwise) until you see the tach drop just a tad and stop turning. Then do the other side. If you have a bigger than stock cam, it may need more gas at idle so after you get both set, try backing each out 1/4 turn at a time for idle quality. Reset the idle speed if necessary.
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Old 09-29-2003, 08:26 AM
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Setting Idle Mixture scews

There are many ways to set the idle mixture screws on a carburetor:
1) By using a vacuum gauge (Goal is to get the highest amount of vacuum at the idle RPM you're trying to obtain)
2) By using a Tach (Goail is to get the engine to idle at the highest RPM with the butterflies as closed as possible)
3) By ear (Goal is to get the smoothest idle quality at the RPM you are looking for)

Any of the above methods will work, so it is best to use which ever way you feel most comfortable with.

A few quick tips:
1) Make sure your engine is in proper working order before adjusting the carburetor. (it won't do you any good to try and dial in the carb if you need to give the engine a tune-up)
2) Make sure the engine is up to proper operating temp before you start adjusting the carburetor. (don't adjust the at 120 degrees temp if you're going to run the car at 165 or 200 degrees)
3) Make sure the ignition timing is set properly (changing the initial timing will change the idle characteristics, if it's too low it will cause the engine to idle too rich)
4) Adjust the mixture screws slowly (the engine pulls the fuel out of the carburetor, so it may take a second to notice a difference. Make your adjustment in or out on the mixture screws, and then rev the engine up a few times, and let it come back to idle)
5) Start with your idle mixture screws out 1-1/2 turns from being seated
6) If you're not sure if going in (leaner) or out (richer) on your mixture screws is making the problem better or worse you can gently press down on the accelerator pump arm without moving the throttle linkage. (This will add fuel without changing the amount of air the engine sees. If it makes the car idle better come out on the mixture screws, if it makes it worse go in on the mixture screws.)
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Old 09-29-2003, 06:38 PM
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wow.. thats perfect.. thanx for the help guys... the motor is in proper working condition and i have given it major tune ups.. the last thing was this carb.. but so far so good.. thanx again
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Old 09-29-2003, 06:52 PM
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Good going, its good to have a great site like this to help each other out.
Plus the good informational replys.
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