Mixture for Holley Carb
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Freshman User
Joined: May 2004
Posts: 35
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From: Melbourne, Victoria
Mixture for Holley Carb
I was wondering the mixture screw settings (for both the screws) for my holley carb. I dont know the model number because i cant find it but it is on a v8 302 F100 72 model can anyone help?
Laughing Gas
Joined: Jul 1999
Posts: 1,048
Likes: 22
From: Lacey, WA
Hi,
You can use the "tried and true" method. Gently turn the screws all the way in. Then back them out 2 turns. Crank the truck and let it come up to operating temp then adjust the screws back in until it starts to bog. Then back out 1/4 turn.
If you have a vacuum gauge do the same procedure while looking for the highest vacuum.
The carb should have some numbers stamped on the front of the choke tower.
Take care,
You can use the "tried and true" method. Gently turn the screws all the way in. Then back them out 2 turns. Crank the truck and let it come up to operating temp then adjust the screws back in until it starts to bog. Then back out 1/4 turn.
If you have a vacuum gauge do the same procedure while looking for the highest vacuum.
The carb should have some numbers stamped on the front of the choke tower.
Take care,
Postmaster
Joined: Feb 2003
Posts: 4,043
Likes: 4
I don't understand what you are asking. Do you want to know WHERE the idle mixture screws are? Or do you want to know HOW to adjust them? Also please post more info... Is it 2 or 4 barrel? Vacuum or mechanical secondary?
Do you think the carb is original or do you know if it has been change or upgraded with an aftermarket model?
You also might try
http://wwww.holley.com
Do you think the carb is original or do you know if it has been change or upgraded with an aftermarket model?
You also might try
http://wwww.holley.com
Postmaster
Joined: Feb 2003
Posts: 4,043
Likes: 4
Go here:
holley.com/HiOctn/ProdLine/Products/FMS/FMSC/f0-7448.html
(Copy the above to your browser address line and add "http://" to the front)
in that picture the very front part of the carb is the fuel bowl. The next part is the main metering block. It is a thin (1/4 inch) rectangular piece. You can see the seams where the metering block meets the carb body and the fuel bowl meets the metering block. The idle mixture screw is on the side of the metering block about 1/4 inch from the bottom of the block. In the picture it's between a red screw that is mounted vertically and the throttle linkage. There is a round recess that the screw is mounted in.
This screw is for adjusting IDLE mixture only! To adjust running mixture you have to dissassemble the carb and replace the main metering jets, and/or the power valve. For more infor on Holley carbs go here:
http://holley.com/HiOctn/TechServ/TechInfo/FMCTech.html
Good luck.
PS why do you want to adjust the mixture (idle or main) of the carb?
holley.com/HiOctn/ProdLine/Products/FMS/FMSC/f0-7448.html
(Copy the above to your browser address line and add "http://" to the front)
in that picture the very front part of the carb is the fuel bowl. The next part is the main metering block. It is a thin (1/4 inch) rectangular piece. You can see the seams where the metering block meets the carb body and the fuel bowl meets the metering block. The idle mixture screw is on the side of the metering block about 1/4 inch from the bottom of the block. In the picture it's between a red screw that is mounted vertically and the throttle linkage. There is a round recess that the screw is mounted in.
This screw is for adjusting IDLE mixture only! To adjust running mixture you have to dissassemble the carb and replace the main metering jets, and/or the power valve. For more infor on Holley carbs go here:
http://holley.com/HiOctn/TechServ/TechInfo/FMCTech.html
Good luck.
PS why do you want to adjust the mixture (idle or main) of the carb?
Last edited by 76supercab2; Jun 18, 2004 at 08:04 AM.
Thread Starter
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Freshman User
Joined: May 2004
Posts: 35
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From: Melbourne, Victoria
THANKS ALOT
I want to change it to make the engine run better at idle.
I got the idle mixture right but now when you tromp the throttle it dies and you ahve to quickly let if off to get back to idle or else it stalls. You can only get to high revs slowly it sounds like its flooding can you help?
I want to change it to make the engine run better at idle.
I got the idle mixture right but now when you tromp the throttle it dies and you ahve to quickly let if off to get back to idle or else it stalls. You can only get to high revs slowly it sounds like its flooding can you help?
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Posting Guru
Joined: Jul 2003
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From: Colville, Washington
Check to see if the accelerator pump is working. With the engine off, pull the air filter and look down the carb throat, move the throttle linkage and see if fuel squirts into the bores. There should be two small streams of fuel with very little throttle movement. The vacuum advance on the dizzy can also cause that problem.
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Laughing Gas
Joined: Jul 1999
Posts: 1,048
Likes: 22
From: Lacey, WA
Fonz,
It sounds like your Accel. pump linkage needs adjustment.
On the drivers side of the carb where the throttle linkage connects you sould also see linkage that goes to the front of the carb. The pump is located under the front bowl. Between the pump and the throttle you sould see a small adjusting screw with a small coil spring under it. This is where you adjust the pump.
You want the pump to start to discharge as soon as the throttle starts to move. Work the throttle by hand and watch the pump linkage. Just don't over adjust the screw or else at WOT the pump will be forced in to far and tear.
The Holley web site has a good explanation of this if my example doesn't make sense.
Good luck,
It sounds like your Accel. pump linkage needs adjustment.
On the drivers side of the carb where the throttle linkage connects you sould also see linkage that goes to the front of the carb. The pump is located under the front bowl. Between the pump and the throttle you sould see a small adjusting screw with a small coil spring under it. This is where you adjust the pump.
You want the pump to start to discharge as soon as the throttle starts to move. Work the throttle by hand and watch the pump linkage. Just don't over adjust the screw or else at WOT the pump will be forced in to far and tear.
The Holley web site has a good explanation of this if my example doesn't make sense.
Good luck,
Postmaster
Joined: Feb 2003
Posts: 4,043
Likes: 4
I agree it sounds like the accelerator pump. The idle screws we discussed at the begining of the thread do not have anything to do with high rpm enrichment. The high rpm enrichment circuit consists of:
1. Accelerator pump
2. power valve
3. main metering jets.
The acclerator pump is on the outside of the carb and the place I would start. The power valve is on the main metering block and you have to remove the fuel bowl to get to it. If the carb has experienced a backfire it can blow out the power valve. I really wouldn't suspect the main jets unless the carb has been swapped.
Another thing to consider is: Is it getting too little fuel as the throttle blades open or is it getting too much fuel. Either condition could cause a bog off idle.
1. Accelerator pump
2. power valve
3. main metering jets.
The acclerator pump is on the outside of the carb and the place I would start. The power valve is on the main metering block and you have to remove the fuel bowl to get to it. If the carb has experienced a backfire it can blow out the power valve. I really wouldn't suspect the main jets unless the carb has been swapped.
Another thing to consider is: Is it getting too little fuel as the throttle blades open or is it getting too much fuel. Either condition could cause a bog off idle.
Thread Starter
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Freshman User
Joined: May 2004
Posts: 35
Likes: 0
From: Melbourne, Victoria
It seems to be getting too much fuel
Becasue it backfires after it bogs down.
Also sorry but i don't know much at all whats the main metering block 76supercab2? and how do i check the power valves condition
Becasue it backfires after it bogs down.
Also sorry but i don't know much at all whats the main metering block 76supercab2? and how do i check the power valves condition
Postmaster
Joined: Feb 2003
Posts: 4,043
Likes: 4
The metering block is the rectangular piec the idle mixture screw is set in. It's also what the fuel bowl mounts to. If you look at the carb or the picture on the holley site I posted you will see fuel bowl - seam- metering block - seam - carb body.
As to checking the power valve the only sure way I know of is to remove it and examine it. I think if you look down the carb throat while running and see raw fuel running into the venturi it could be the power valve. (But it could also be a misadjusted float or stuck needle valve). Does anyone else have tips on diagnosing a bad power valve?
As to checking the power valve the only sure way I know of is to remove it and examine it. I think if you look down the carb throat while running and see raw fuel running into the venturi it could be the power valve. (But it could also be a misadjusted float or stuck needle valve). Does anyone else have tips on diagnosing a bad power valve?
Laughing Gas
Joined: Jun 2003
Posts: 1,166
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From: Chino, California
The power valve is mounted in the metering block. If you can pull off the fuel bowl, and then the metering block you can see if the PV is not seated. If it's seated, it's probably okay. If it's not seated, then it's stuck open, which means it's bad.
However, "bogging" on acceleration, (if it's not timing) is generally one thing....the accelerator pump is either coming in too late or not at all. It's an easy fix. The diaphragm is located on the bottom of the fuel bowl. Just look down the throats of the carb while manipulating the linkage. If you don't see two healthy streams of fuel shooting into the venturi's, replace the pump diaphragm.
There's one more thing to consider.....backfiring sounds like distributor problems. Check the vacuum advance diaphragm and be sure the arm is moving the distributor's breaker plate. You'll have to suck on the vacuum line to the distrib and look at it with the cap removed to see if there is movement. If you suck air, the diaphragm is perforated...get a new one.
However, "bogging" on acceleration, (if it's not timing) is generally one thing....the accelerator pump is either coming in too late or not at all. It's an easy fix. The diaphragm is located on the bottom of the fuel bowl. Just look down the throats of the carb while manipulating the linkage. If you don't see two healthy streams of fuel shooting into the venturi's, replace the pump diaphragm.
There's one more thing to consider.....backfiring sounds like distributor problems. Check the vacuum advance diaphragm and be sure the arm is moving the distributor's breaker plate. You'll have to suck on the vacuum line to the distrib and look at it with the cap removed to see if there is movement. If you suck air, the diaphragm is perforated...get a new one.