Carb Tuning - Vintage Mustang Forums

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post #1 of 26 (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 26 (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 26 (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 26 (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 26 (permalink) Old 02-16-2017, 10:14 AM Thread Starter
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Thanks!
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post #6 of 26 (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 #7 of 26 (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 #8 of 26 (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 #9 of 26 (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 #10 of 26 (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 #11 of 26 (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 #12 of 26 (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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post #13 of 26 (permalink) Old 02-17-2017, 11:11 AM
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https://www.amazon.com/Holley-Carbur...buretor+tuning

The first edition of this back in the mid late 60s is were I started learning about carbs suggest reading it several times

I still pull it out now and then to refresh my memory as it describes how some circuits work a bit different than others in different style carbs.

s far as a Holley style idle circuit goes the idle mixture is actually controlled by the Idle Air Bleeds (IAB). The air pulled through these mixes with fuel pulled through Idle Fuel Restrictions (IFR). The Idle Feed Screws adjust the amount of the emulsion (the mixture of air and fuel from the IAB and IFR. [The idle mixture screw in most carbs dosent really adjust the mixture just the amount].
The mixture is realy closest to being correct when the idle feed screw set at best a half turn in or out compromises the idle. Idle fuel comes out a small hole below the throttle blade KNOWN AS THE Idle Feed. Just above the Idle Feed Hole is the Idle Transfer Slot. About .030 to .045 of that slot should be showing between the blade and the idle feed hole. The main jet dosent come into play in the idle circuit unless main jet sizes change by 10 sizes or more.

The idle circuit transfers into the main circuit around 2000 RPM give or take in most cases. IABs have some control as to when the idle circuit transfers to the main circuit so changing their sizes affects the transition point.
The IFR has the most effect on the idle fuel emulsion as it measures the fuel like the main jet. The IAB makes changes in much smaller steps.
Big issue except in certain carbs drilling out these orifices have to be drilled out or filled in then redrilled or in dome cases a piece of guitar wire used, holes drilled and tapped for size 6 or 8 brass set screws to be drilled out and used as jets inplace of the brass inserts.
Or a carb such as the Quick Fuel carbs that have removable jets in all metering orifices.
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post #14 of 26 (permalink) Old 02-17-2017, 12:41 PM
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Quote:
Originally Posted by turbo2256b View Post
"....... Or a carb such as the Quick Fuel carbs that have removable jets in all metering orifices.
Also good to know: the mid range & higher end Holley's such as the Street HO series have additional tuning ability, like replaceable air bleeds. But it bears pointing out, tht unless your engine is greatly modified, the basic Holley will work just fine. Extra adjustability doesn't always mean superior performance.




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post #15 of 26 (permalink) Old 02-17-2017, 01:10 PM
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Four corner idle, substituting air bleeds, etc. are all things that are more
the bailiwick of someone tuning on a dyno or who has access to a wide
band A/F meter (like an Innovate) at the very least.
Without those things you're just guessing at fuel curve.

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