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post #1 of 33 (permalink) Old 02-16-2017, 01:22 AM Thread Starter
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Carb Tuning

Recently I have been wanting to learn more about how to fine tune my engine. Naturally, I want to start with the carb.

My question is straight forward. How do the idle mix screws interact with the jets? Do the idle mix screws only control the idle circuit (no throttle)? And the jet size controls throttle?

For instance, if my a/f was 12-12.5 at 25-50% throttle and I wanted to lean out the system would adjusting the idle mixture screws be the wrong approach? Should I be changing jet sizes instead?

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post #2 of 33 (permalink) Old 02-16-2017, 06:29 AM
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idle mixture will affect the overall mixture, but minimally.

trying to lean out the whole system as you describe via the idle mixture screws is not an effective way to do so.

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post #3 of 33 (permalink) Old 02-16-2017, 08:40 AM
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Quote:
Originally Posted by McKay View Post
My question is straight forward. How do the idle mix screws interact with the jets? Do the idle mix screws only control the idle circuit (no throttle)? And the jet size controls throttle?

For instance, if my a/f was 12-12.5 at 25-50% throttle and I wanted to lean out the system would adjusting the idle mixture screws be the wrong approach? Should I be changing jet sizes instead?
You should be changing the jet size. The idle mixture is the first thing you set when you start tuning your carb. If you don't change your idle speed, timing, etc then you should leave your idle mixture alone. You want somewhere around 13.5 -14 air fuel ratio on your idle. If you drive your car a lot through different temps, altitudes, etc and don't want to ever touch your idle mixture again then perhaps you would want to be on the rich side of that.

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post #4 of 33 (permalink) Old 02-16-2017, 09:04 AM
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Here's a good resource for you; http://www.edelbrock.com/automotive/...411_manual.pdf

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post #5 of 33 (permalink) Old 02-16-2017, 10:14 AM Thread Starter
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Thanks!
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post #6 of 33 (permalink) Old 02-16-2017, 10:26 AM
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The idle mixture screws are completely separate from the jets. The idle circuit gets it's fuel without the function of the main jets coming into play at all. When there is not enough air flow through the venturi, no fuel is drawn from the nozzle, which is fed by the main jets. Therefore, engine vacuum is the only source for pulling fuel to the engine. Since the throttle plate is practically closed, you need a fuel exit below the throttle plate.. That is where the idle port is. That port is controlled by the idle mixture screws. It functions this way until enough air moves through the venturi at which point the idle port means practically nothing. basically, it still functions to about 25-30 MPH, depending on the gear ratio of your rear. It functions less and less once you get to the transition point where the transition slot and idle port are not doing nothing. The transition slot sits higher in the barrel, so it functions a bit longer or later, but again, all gone by 30 MPH or so.
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post #7 of 33 (permalink) Old 02-16-2017, 10:59 AM Thread Starter
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Thanks again for the explanations.

I wanted to thin out the system and so I started messing with the mixture screws. Then it ran poorly (not awful just a little rougher than I would have liked) which got me thinking about the idle mixture screws an how they interact with the jets.

Low and behold, my suspicions are correct and I'm tuning it incorrectly.
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post #8 of 33 (permalink) Old 02-16-2017, 11:34 AM
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Not trying to advocate for another forum by any means, but I found this thread really helpful in understanding the carb fuel circuits.

Scroll about 1/2 - 3/4 the way down on the first page, and you get some really nice diagrams

autolite 4100 Page1 - Mustang Monthly Forums at Modified Mustangs & Fords Magazine


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post #9 of 33 (permalink) Old 02-16-2017, 11:49 AM
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You need to figure out what circuit is in play in the area of operation you want to make leaner. At 25% throttle opening you may be on the transition slot(idle circuit) and not the mains.

The transition slot gets fuel from the main well through a calibrated orifice called the idle feed restrictor (ifr). The ifr also feeds the idle discharge holes which are metered by the mixture screws. The t-slot and the idle discharge flow until the velocity of air through the main well is such that the pressure in the main well becomes lower than the vacuum present at the t-slot. Once this occurs (and contrary to popular belief) the t-slot begins to flow backward introducing air into the main well effectively acting as a main air bleed and is how the idle circuit calibration can influence main circuit calibration.

Edit: The above applies to a Holley style carb which is what I assumed the OP was working with.

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post #10 of 33 (permalink) Old 02-16-2017, 12:03 PM
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Originally Posted by McKay View Post
Thanks again for the explanations.

I wanted to thin out the system and so I started messing with the mixture screws. Then it ran poorly (not awful just a little rougher than I would have liked) which got me thinking about the idle mixture screws an how they interact with the jets.

Low and behold, my suspicions are correct and I'm tuning it incorrectly.
There is a correct way to do this, anything else will yield bad results.You need a properly warmed up engine. You need your timing set. It is best to have a vacuum gauge as well. can you do this?
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post #11 of 33 (permalink) Old 02-16-2017, 12:48 PM Thread Starter
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Originally Posted by Claudemiro View Post

There is a correct way to do this, anything else will yield bad results.You need a properly warmed up engine. You need your timing set. It is best to have a vacuum gauge as well. can you do this?
Yep, but isn't an a/f meter enough? Or do I have to use a vacuum gauge as well?
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post #12 of 33 (permalink) Old 02-16-2017, 12:59 PM Thread Starter
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Huh, I didn't know there was a transition circuit. I only knew about the idle and main.

To the books for more studying!
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post #13 of 33 (permalink) Old 02-16-2017, 01:28 PM
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If you have a Holley you can get different front plates to adjust A/F for different rates of vacuum. Either tune able plates or some from the smog era where the by-product were lean/efficient areas on their spectrum.
With all the right gauges you could find your sweet spots of but its always a compromise + staying safe, running too rich, costs less than too lean
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post #14 of 33 (permalink) Old 02-16-2017, 02:46 PM
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Huh, I didn't know there was a transition circuit. I only knew about the idle and main.

To the books for more studying!
The transition circuit is what controls things as you move from off idle to the main circuit. It is still effected by the idle mixture screws, but the main jets begin to do "something" during that time as well. It's a low speed circuit.
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post #15 of 33 (permalink) Old 02-17-2017, 05:10 AM
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Tuning a carb is very complicated, but doable. It depends how far you are willing to dig to do it right.

There are basically two circuits on the typical Holley (low speed and high speed). As was stated before, the idle resides on the low speed circuit.

If you wanted to lean out part throttle, you will need to run a smaller IFR. Most aftermarket Holley metering blocks run .036 IFR's. I found a .032 or so is more acceptable, assuming you have decent vacuum. But that only has a limited result in leaning out the part throttle. You don't want to go too small with the IFR because your carb pulls fuel from the transition slots during cruise and very light throttle, and those would run lean too with the change.

There is more to be done to lean out the part throttle. I'm assuming you have set your main jets for a good WOT AFR (~12.7-13.1), and a correct power valve for your vacuum (and capped rear power valve). The next step would be to run larger power valve restrictors so that you could run a smaller main jets on the front metering block. This way you still have the same overall jet needed for 3/4 throttle - WOT, but a smaller leaner jet for soft part throttle.

Having said all that, I doubt you have adjustable metering blocks.

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