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Discussion Starter #1
I want to lean out the primary's just a bit on a QF-Q850. Can I go up 2-4 size in high speed air bleed on the primary only? Or will that mess up the "balance" of the carb at WOT? Trying to lean it out a touch 2700-3400 at cruise..... Jetting on the AFR @ WOT is pretty close.
 

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I want to lean out the primary's just a bit on a QF-Q850. Can I go up 2-4 size in high speed air bleed on the primary only? Or will that mess up the "balance" of the carb at WOT? Trying to lean it out a touch 2700-3400 at cruise..... Jetting on the AFR @ WOT is pretty close.
They way I understand it, you would be going in the wrong direction by going up in bleed size because that's going to lean out the top where you are ok now. My guess is that it will need a reduction in emulsion air.

It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.
The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.
 

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Discussion Starter #7
I should add that the car has forward ram air. 2-3" ducts on the nose to a airbox. The faster it goes,the leaner it gets.....
 

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They way I understand it, you would be going in the wrong direction by going up in bleed size because that's going to lean out the top where you are ok now. My guess is that it will need a reduction in emulsion air.

It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.
The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.
Great detailed post.
 

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you need to get a vacuum reading at the cruise rpm you described.
then note about how much your into secondaries .if at all ..there are 3 links available and different progressives
then you can size the pv. to hold back fuel at that point .
if ist already closed off then you can lean on the primary jet and add it back into the PVCR
the metering jets/holes behind the power valve.
with this data you can nail the drivability you need with your given carb size and signal pull.
i have had cars need a 14 jet spread and others that wanted 7
 

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power valve channel restrictors. They will drill the tiny holes in the plates behind ur powervalve to fatten up the idle a bit or restrict whatever is needed. Blowthrough guys use that trick
 

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upping a pvcr wont effect idle .its blocked off by pv and feeds the same main well as the jet .
it will only effect enrichment over base jets when open
 

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Discussion Starter #12
The idle mixture is fine. 12.4-12.6 AFR, it just drops to 11.2-11.5 driving..want to lean that out a touch......is it easier to pull jet out of the front and add he same amount to the secondaries to compensate for WOT?? It is not in the secondaries under cruise..... Thought adding a 2size high bleed would do the same, just didn't know how it would effect the "balance" of the carb if there are different hi bleeds in front vs back....
 

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Before you make changes try building a bowl vent crossover like any one in the following link. If that has no effect my suspect is you will need to go down on the main air bleed, try some .028's, and you will need to go down on jets as well. I'm figuring around 82-83 with the bleed change for the secondary, 74-75 in the primary.
 

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Originally Posted by n5ifi
They way I understand it, you would be going in the wrong direction by going up in bleed size because that's going to lean out the top where you are ok now. My guess is that it will need a reduction in emulsion air.

It is well documented that introducing air into the main well encourages low signal flow and can encourage or discourage high signal flow. The natural characteristic of a plain jet and nozzle (no air) is to get richer as airflow increases. The purpose of the air bleed system is to modify that behavior to accomplish a constant (or the desired) air/fuel ratio over as wide a range of airflows as possible. The particular ratios for power and cruise are realized by the selection of jet and rod or jet and auxiliary jet (power valve channel). The purpose of air bleeds is not to emulsify but to accomplish the correct fuel delivery. Emulsion is just a beneficial side effect.
The vertical location of the bleeds entering the main well influences the fuel flow in the following ways.

1: Orifices above float level or between the well and the nozzle allow bled air to raise the pressure (reduce the vacuum) in the nozzle and above the fuel in the well. That delays the initial start of fuel flow from the nozzle to a higher air flow through the venturi and is used to control the point in the early throttle opening where the main starts.

2: Orifices at float level increase low range (early throttle opening) fuel flow by carrying fuel with the airflow to the nozzle.

3: Orifices below float level increase fuel flow by the effect of lowering the level of fuel in the well to the hole(s) admitting air. This is like raising the float level a similar amount (increases the effect of gravity in the pressure difference across the main jet) and also adds to the airflow carrying fuel to the nozzle. Locating the orifices at different vertical positions influences this effect’s progression.

4: The "emulsion holes" influence is greatest at low flows and the "main air bleed" has most influence at high flows.

In the first three cases above, once fuel flow is established it is greater than it would be with fewer or smaller holes. Visualize wind blowing spray off of the top of water waves. It doesn’t take much pressure difference to cause the velocity of the airflow through the bleed orifices to have significant velocity in the orifice, even approaching sonic (1100 F.P.S.) if the orifices are small. The phenomena of critical flow is what limits the total air flow through an orifice and allows tuning by changing bleed size.

Essentially, the emulsion effect will richen the low flow and the air bleed size, main well and nozzle restrictions will control the increase or reduction of high flow. Again, the desired air/fuel ratio is the primary purpose of the bleed system. "Improved emulsion" is an oxymoron if the modification of air bleeds to "improve emulsion" results in an incorrect air/fuel ratio in some range of engine operation. Correct proportioning of all the different bleeds (and, of course, the idle, transition and power circuits) will give the correct air/fuel ratios over the total range of speeds and loads and a flat air/fuel ratio characteristic at wide open throttle.
Great detailed post.
He copied it from here
http://racingfuelsystems.wonko3.myfunforum.org/viewtopic.php?p=118#p118
 

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Before you make changes try building a bowl vent crossover like any one in the following link. If that has no effect my suspect is you will need to go down on the main air bleed, try some .028's, and you will need to go down on jets as well. I'm figuring around 82-83 with the bleed change for the secondary, 74-75 in the primary.
It would have helped to include the link. :rolleyes:

http://racingfuelsystems.wonko3.myfunforum.org/viewtopic.php?f=25&t=15

The richness at level road cruise is likely because the Low Speed Circuit is too rich. The Low Speed Jet is commonly called the Idle Jet, but it actually controls the steady speed level road mixture and light acceleration up to about 3,000 RPM or more.

The IJ and IAB are what you need to work with (besides everything else). Making the main air bleed larger will make the small throttle opening of level road cruising richer than it already is. You see why Mark advised a smaller MAB.

 

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It would have helped to include the link. :rolleyes:

http://racingfuelsystems.wonko3.myfunforum.org/viewtopic.php?f=25&t=15

The richness at level road cruise is likely because the Low Speed Circuit is too rich. The Low Speed Jet is commonly called the Idle Jet, but it actually controls the steady speed level road mixture and light acceleration up to about 3,000 RPM or more.

The IJ and IAB are what you need to work with (besides everything else). Making the main air bleed larger will make the small throttle opening of level road cruising richer than it already is. You see why Mark advised a smaller MAB.


It wasn't my intention to make it mine.
That's why I had two different colors in print.
I should have posted the link for sure so my bad.

Fact is I am very interested in all these carb discussions too so was I in the ballpark or was something else in play that I didn't think of?

Of course I have learned most everything I know of a technical nature from Jmark ,Yeti and Tuner.

I'm in no way an expert but I'm not stupid either. I've modified several carbs now from the stuff I learned from you guys and they are much better than they were before the mods.
 

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Before you make changes try building a bowl vent crossover like any one in the following link. If that has no effect my suspect is you will need to go down on the main air bleed, try some .028's, and you will need to go down on jets as well. I'm figuring around 82-83 with the bleed change for the secondary, 74-75 in the primary.
Why the vent crossover?

Won't going down on the MAB size to .028 from .033 make it richer at high speed ( full throttle) or at least more rich than it is now?
 

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It would have helped to include the link. :rolleyes:

http://racingfuelsystems.wonko3.myfunforum.org/viewtopic.php?f=25&t=15

The richness at level road cruise is likely because the Low Speed Circuit is too rich. The Low Speed Jet is commonly called the Idle Jet, but it actually controls the steady speed level road mixture and light acceleration up to about 3,000 RPM or more.

The IJ and IAB are what you need to work with (besides everything else). Making the main air bleed larger will make the small throttle opening of level road cruising richer than it already is. You see why Mark advised a smaller MAB.

Oh, That's not where I got the information from but good link.


Does it make his carb richer because of the already to rich idle circuit at part throttle?


I was under the impression that the bigger MAB helped to delay the main circuit start? I have seen in some threads that it does make the 2500-3500 richer but I'm not sure why unless that's has something to do with to much emulsion air.
 

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Bowl vent tube in the event the vents are seeing turbulence, it will make it lean up top. I'm betting it's a bleed issue, but it never hurts to try a bowl vent crossover.
 
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