fuel consumption vs afr ratio
Thread Starter
Racer
Joined: Jul 2008
Posts: 323
Likes: 18
You are correct in stating that you can fine-tune the AFR at light throttle cruise by using a restrictor wire in the idle feed restrictor orifice. What I have found works perfectly in many cases is a simple staple out of an office stapler: Pop 2 staples out of a stapler and straighten out one of the bends so you have an "L" shaped piece of wire. Stick the short leg of the "L" into the IFR, and sandwich the long leg under the metering block gasket for retention. This will quite often get the cruise mixture up from mid-12's into the mid-14's, and it will also make your idle mixture screws more finely tuneable (will change idle mixture screw settings from 3/4 turn out to 2 turns out, as an example).
Lars
Lars
Le Mans Master
Joined: Mar 2004
Posts: 7,503
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From: Little Rock AR
The wires in the metering block reduce the fuel flow through that opening. Larger air bleeds increase the air flow thru that opening.
This is the circuit you are modifying to better control the mixture. Its just air and fuel. You can try bigger air bleeds and/or smaller fuel passages. If you lean one or both of these 2 out, the adjustment screws can trim the last part of the mixture better. After the mod(s) the screws will be providing a bigger portion of the total fuel flow in this circuit. Gives you more control.
The wire mod is fast, cheap and easy as compared to messing with bleeds. It gets the job done. You do not need to pull your hair out replacing and tuning bleeds if all it takes is a couple of staples in the metering block.
This is the circuit you are modifying to better control the mixture. Its just air and fuel. You can try bigger air bleeds and/or smaller fuel passages. If you lean one or both of these 2 out, the adjustment screws can trim the last part of the mixture better. After the mod(s) the screws will be providing a bigger portion of the total fuel flow in this circuit. Gives you more control.
The wire mod is fast, cheap and easy as compared to messing with bleeds. It gets the job done. You do not need to pull your hair out replacing and tuning bleeds if all it takes is a couple of staples in the metering block.
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
Yes increasing the low speed air bleeds will lean the idle and cruise but may also change the fueling curve
If your carb has 4 corner idle then do all 4,
Neal
If your carb has 4 corner idle then do all 4,
Neal
Last edited by chevymans 77; Jul 27, 2020 at 10:31 AM.
Thread Starter
Racer
Joined: Jul 2008
Posts: 323
Likes: 18
Just because I Loke to learn stuff, one more question
If you have a carb with air bleed jets AND restriction jets at the metering block. To get the same result, What is the best thing to do, bigger air bleeds or smaller restrictions? What does an engine perfer between thoose two options?
If you have a carb with air bleed jets AND restriction jets at the metering block. To get the same result, What is the best thing to do, bigger air bleeds or smaller restrictions? What does an engine perfer between thoose two options?
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
It is best to decrease the idle feed restriction than to increase the idle air bleed within reason meaning that if changing the idle air bleed a very small amount gets you where you need to be then that would be acceptable but making changes that would alter the fuel curve will cause issues unless you make other changes to get it back inline.
Neal
Neal
Le Mans Master
Joined: Mar 2004
Posts: 7,503
Likes: 1,511
From: Little Rock AR
Here is a generic diagram that explains how this idle feed circuit works. You would want to add a wire to the idle feed restriction "idle jet" in the diagram to reduce fuel flow.
Holley's look a little different but the concept is valid.
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
I doubt you need to play with air bleeds. If you do go that route, air bleed changes are like 10 sizes up or down to make any difference whereas main jets are one or two. Same with the idle restrictors.
Last edited by resdoggie; Jul 27, 2020 at 04:28 PM.
Safety Car
Joined: Nov 2005
Posts: 3,961
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From: Lompoc, CA. Santa Barbara County
I run a Holley Sniper fuel injection on my 427 LS3, so this may not be of any help. But I set my idle at 13.8, cruise at 14.2, and WOT at 12.5. These 11 to 1 CR LSs need a rich WOT I'm told. After decades of fine tuning that Q-jet, it's fun just punching the AFR into the handheld. My main reason for posting is that 12.4 for cruise is way too rich.
Thread Starter
Racer
Joined: Jul 2008
Posts: 323
Likes: 18
I run a Holley Sniper fuel injection on my 427 LS3, so this may not be of any help. But I set my idle at 13.8, cruise at 14.2, and WOT at 12.5. These 11 to 1 CR LSs need a rich WOT I'm told. After decades of fine tuning that Q-
jet, it's fun just punching the AFR into the handheld. My main reason for posting is that 12.4 for cruise is way too rich.
jet, it's fun just punching the AFR into the handheld. My main reason for posting is that 12.4 for cruise is way too rich.
Melting Slicks
Joined: May 2002
Posts: 2,717
Likes: 121
From: Sulphur LA
St. Jude Donor '05-'06,'11,'13-'14,'16,'18,'19,'24, '25
actually air bleeds help define the sensitivity of the main restrictions, main jets, IFR, etc....
Depending on the carb and the size of the air bleeds a small move can make a large change
Generally speaking it takes about 10 main jet sizes to move the AFR 1 full point,
As I mentioned air bleeds are a good fine tuning option
Neal
Depending on the carb and the size of the air bleeds a small move can make a large change
Generally speaking it takes about 10 main jet sizes to move the AFR 1 full point,
As I mentioned air bleeds are a good fine tuning option
Neal