How to adjust carburetor mixture screws

Here's how to find and adjust the mixture screws on your carburetor.

Note that metering-block style carbs such as Holley and Demon more often have one or 2 mixture screws in each metering block depending on the number of venturi. Also those may not be located at the bottom of the carb as in factory carburetors.

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• varajet
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• carburetor
• adjust
• mixture
• screws
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• gas

[80v8camaro]

Is better a vaccum guage or a Air/fuel ratio guage for tuning ??

[WhoSaidTyler]

@80v8camaro For idle mixture just use a vacuum gauge. Tune for the highest vacuum reading.

[manlymud]

i have a question. when he was talking about getting the screws so its the fastest, so the more you screw it in. the more mpg, the more you unscrew the more horsepower??? is that correct?

[WhoSaidTyler]

@manlymud No. The idle mixture screws only control the mixture at idle, has nothing to do with mpg or power. What it means is turn the screw one direction or the other until the idle speed is the fastest. No set spot, just experiment. The engine slows down when you go too far either direction.

[apaulk86]

But I do have one question: My choke doesn't seem to be setting when I start the truck. Is there something I have to do so the choke will set? I am sure the two of these are related, but also, the engine won't rev high when it's cold, and it kind of chugs for a minute or two until it starts to warm up. Any suggestions?

[WhoSaidTyler]

@apaulk86 check my choke videos, they'll be of help. It sounds like the choke spring is broken or out of adjustment.

[dlfranci]

Great video, thanks! I do have a couple questions though. The highest vacuum reading that I can get is 15" and that is about 5-6 revolutions open from starting at all the way closed on both idle mixture screws, but when I do this it seems to run poor. If I take it down to 13" which is 4 revolutions open it runs a lot better. Why is it running better on 13" as opposed to 15" and secondly, does 4-6 revolutuions from open seem too rich, or would that seem reasonable?

[Hot80s]

i bought a 57 210 coupe that sat 10 years in the mud it has a 350 with a Q-jet. i got it running, turned down the fast idle, but when she gets a little warm (outside temp 40) & gets off the 1st notch (half) of choke it starts to make a mild backfire out the left side intermitently. If i push the choke armdown all the way she wont idle warm off choke. I havent touched the quads mixture screws yet. i watched your video, thanks. any other advise?

[L6292]

I'm pretty sure you want to be below the throttle plate when reading vacuum.

watch?v=vh3Z-F6KGkI&feature=ch­annel_video_title

[titoli1]

how do i know what mixture is good. even if both cylinders run equal

[689321546]

@MaxMatt2 ...hi the Q-JET has a serial number thats how you would order the right rebuilt kit . as for the electric choke it would be a good idea , the choke thats on there now its a hot air pickup type thats has a tube connect to the manifold correct ! if you were not so far away i send you a motor manuel for your year car .i have rebuilted many Q-JETS in my day even the best of rebuild's the carb still run's bad it the way the carb was made .from day one . inline filters dont forget

[MaxMatt2]

@689321546 It's a 1971 Cadillac Eldorado. I see many kits on ebay, but have no idea which one is good. I keep the car as a classic, so I want all the best for it. Is it worth it to upgrade to electric choke? Right now I am sorting out the ignition system and vacuum lines. Thanks for feedback!