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Old 11-04-2010, 09:55 AM
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Proper adjustment of idle screws.

This is just a general question that I was wondering about and hope someone can help with an answer.

I understand how Holleys and other Holley style carbs....quickfuel, demon, bg ect work.

Its well known how to set the idle transfer slots and set the idle mixture screws. Starting at 1 1/2 turns out and adjusting for maximum vacuum etc etc.


What if the car needs the idle mixture screws turned OUT 1/2 turn to get highest maximum vacuum.
OR
What if the car needs the idle mixture screws turned OUT 3 1/2 turns out to get highest maximum vacuum.

Is there a such thing as too far in or too far out if the the screws are responsive and the car idles perfect?
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Old 11-04-2010, 10:12 AM
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Doesn't matter where they are. 1 1/2 is a baseline for a new install and you adjust from there. Mine are a good 3 1/2 out due to my setup. So long as they do have an effect (rpm drops or increases) they are working. Often, another 1/2 turn out is needed even after highest vac for some setups that need a bit richer idle.
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Old 11-04-2010, 10:32 AM
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bubba is right, though I would be sure to check your initial timing and idle adjustment, needing excessive fuel can be an indicator of something wrong here, or it could just mean you have a rowdy engine that has poor idle characteristics.

In any event it needs what it needs.
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Old 11-04-2010, 11:05 AM
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If the holley carb idle screws need to be 3-1/2 turns out from seated, to get the best idle, I'd be looking for a carb or manifold vacuum leak.
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Old 11-04-2010, 01:02 PM
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Quote:
Originally Posted by F-BIRD'88
If the holley carb idle screws need to be 3-1/2 turns out from seated, to get the best idle, I'd be looking for a carb or manifold vacuum leak.

So if 3 1/2 turns out indicate a possible vacuum leak.

What does a carb with the idle screws out 1/2 to 3/4 turns out indicate?
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Old 11-04-2010, 09:19 PM
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Every carb manual has a similar procedure for idle air screw adjustment.

As long as your initial timing and idle RPM are correct for your application, set the screws for the best idle quality using the leanest possible setting. Connect the vacuum can of the distributor to manifold vacuum to get as much advance at idle for best fuel economy which will also keep engine idle temps down and you won't foul plugs.

There is no value in having an excessively rich idle.

Use a vacuum gauge and check your vacuum at idle and note needle quality, steady, wobbly, erratic, etc. Check it out here and report back, 3-1/2 turns does not indicate a vacuum leak by virtue of the number of turns but may indicate something to look for.

Last edited by octanejunkie; 11-04-2010 at 09:24 PM.
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Old 11-04-2010, 10:10 PM
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octanejunkie............

Thanks for the information, but I think you may have misunderstood my question. I understand how to adjust the idle screws with the use of a vacuum gauge, its a very simple procedure.

I was wondering if there is a such thing as having the screws adjusted in too far or out to far and the effects of each.

Let say someone has their car setup and the idle screws are out 1/4 turn from seat and it runs great..........should they just be happy or does this indicate the need to change air bleeds to get the screws adjusted out further.

Same goes for if someone is say 3 1/2 turns out from seat. Should they be happy it runs great or should they adjust the air bleeds to get the screws turned in more.
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Old 11-04-2010, 10:28 PM
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My 2 cents.

I find range where idle is good. Say 1 to 2 turns out is good.

I run at the leaner 1 turn as long as no problem w/ headers overheating, etc.

My idle cleans plugs to white.

This is very useful when tuning cruise mode, 2bbl mode, wot mode of carb via plug readings.

Once carb is tuned, I may tune idle a bit richer so plugs have a little color.
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Old 11-04-2010, 11:02 PM
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Quote:
Originally Posted by 427blownvette
Let say someone has their car setup and the idle screws are out 1/4 turn from seat and it runs great..........should they just be happy or does this indicate the need to change air bleeds to get the screws adjusted out further.

Same goes for if someone is say 3 1/2 turns out from seat. Should they be happy it runs great or should they adjust the air bleeds to get the screws turned in more.
That depends on whether someone is happy that their car runs great or whether they like to find things to fix

As far as addressing your real question, as you know, every build is different and there are many variables that affect optimal settings for each. I think the better question would be to want to know WHY one runs great one way and the other runs great another way, not how to equalize them...
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Old 11-04-2010, 11:33 PM
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Quote:
I was wondering if there is a such thing as having the screws adjusted in too far or out to far and the effects of each.
There is no constant, maybe a bench mark,,,get them actually doing something when adjusted and remeber the turns out at max vacuum, each motor needs what it needs, as long as your actually pulling on the circuit at idle who cares how many turns.
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Old 11-05-2010, 09:09 AM
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They wouldn't make it an "idle adjstment" if it wasn't meant to be custom adjusted for each setup. Too far in is only wrong if it's maxed in abd having no effect.
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Old 11-05-2010, 08:06 PM
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Make sure the timing is set and the engine warmed and up to operating temp, set the mixture idle mixture screws by tapping on the accelerator pump to give the engine a quick shot of fuel, but without cracking the throttle open. If the engine speeds up, the mixture is lean, if it stumbles, the mixture is rich. If there is no change, you're pretty much there (I also use see through plugs to see the float levels). Adjust the mixture screws equally every time by 1/8 to 1/4 turn, allow a few moments up to a minute for the carb to adjust to the settings. Move to the secondary side if you have 4, doing the same thing. Adjust both/all mixture screws at once, but with smaller adjustments. I also use the vacumn method for the street to tame my camshaft which brings in about 2 full turns. Ideal situation is placing it on a monitor and adjusting with the electronic line graph. (ie. Autotune; Firestone; or the Stealer.) Drive and check plugs.
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Old 11-05-2010, 08:55 PM
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Quote:
Originally Posted by aintmisbehavinn
Make sure the timing is set and the engine warmed and up to operating temp, set the mixture idle mixture screws by tapping on the accelerator pump to give the engine a quick shot of fuel, but without cracking the throttle open. If the engine speeds up, the mixture is lean, if it stumbles, the mixture is rich. If there is no change, you're pretty much there (I also use see through plugs to see the float levels). Adjust the mixture screws equally every time by 1/8 to 1/4 turn, allow a few moments up to a minute for the carb to adjust to the settings. Move to the secondary side if you have 4, doing the same thing. Adjust both/all mixture screws at once, but with smaller adjustments. I also use the vacumn method for the street to tame my camshaft which brings in about 2 full turns. Ideal situation is placing it on a monitor and adjusting with the electronic line graph. (ie. Autotune; Firestone; or the Stealer.) Drive and check plugs.
Hadnt heard of the accel pump only trick myself. Going to try it tomorrow. Youre saying to dump the accel pump fuel onto the top of the plates? Push the accel pump arm down quickly by hand without actually giving it throttle is going to shoot gas onto the top of the primary blades
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Old 11-05-2010, 09:58 PM
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BlownVette?

Hmmm the OP's handle has me wondering about the specific idle circuits on blower carbs, is there a difference between these and say standard holleys? just ask'n thanks
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Old 11-08-2010, 03:13 AM
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something interesting I ran into after a LOT of digging on holley's website.

I had a 600 vac. sec. carb I couldn't get control of the idle on for NOTHIN..

I found a totally different procedure for setting the idle that worked

step one is to remove the carb from the engine.

and adjust the idle mixture screw until a certain amount of the idle transfer slot was exposed.. (I THINK it was .060" but I could be wrong on the number)

put the carb back on and attempt the start the engine.

if it idled way to fast or not fast enough, remove the carb, and adjust the _SECONDARY_ idle stop, which is a small hex-key screw on the bottom of the base plate to get the idle speed close.

once that was dialed in close, set the mixture screws with a vac. gauge and use the SECONDARY idle stop to adjust idle speed.

after doing that. ( it took a few tries) I finally got the idle circuit working the way it should and was able to adjust the idle speed with the primary adjuster on the side of the carb as needed (michigan winters call for a slightly richer idle mixture)
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