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Old 05-11-2009, 09:41 AM   #1
urban_cowboy
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Default Tuning With Air Bleeds

As most of you know, I have built out a 383 stroker producing about 500ft*lbs of torque. I have been offered a Saturday tune session on a chassis dyno, which got me motivated to finish my tuning so that I can refine it on the dyno. While doing that last week, I ran into several obsticals with my Holley 750 double pumper that lead me to purchase a Holley 750HP.

I received it on Friday and immediately wrote down the inital settings of all the critical components. I then replaced the primary and secondary jets and primary air bleeds to match an article I recently read about Holley HPs and FSB strokers. I put the carb on and started the engine. I set the idle speed to 900rpm and set the idle needles so that idle AFR was about 13.5-14. Like the double pumpers, the primary throttle valves were open a lot which caused dieseling once the engine got warm. I knew from experience that opening the secondary throttle valves about 1/4 -3/8 of a turn would even the air flow out between the front and back throttles at idle which helped eliminate the dieseling. I did not have time to drive the car, but from working the throttle out of gear, it appears that the primaries are about AFR from idle to 3500. At that point they start to go rich.

Here I come to my questions. The reason I purchased the Holley HP was to get a carb with adjustable air bleeds, which appears to be what I need to change to refine the turn on this car. I have searched and search but found very little info about tuning procedure with air bleeds.

1. At what point do you change jets and at what point do you change air bleeds? They both affect the AFR of the high speed circuit.

2. At what point do you change air bleeds on the speed circuit? The air bleed and idle screw both affect the AFR.

3. Should the primary and secondary idle or low speed air bleeds be the same? They were not out of the box, but given the throttle position by engine likes, air flow is basically the same through the front and back at idle. I would think it would stand to reason that the air bleeds would be the same and the idle screws would be the same all the way around (or at least very close).

4. I know the air bleeds add air into the circuit which leans that circuit out. I also understand that is delays the operation of that circuit but reducing the signal...I cannot grasp this concept. How can the idle circuit be delayed?

What I say with my other carb is at cruise @3300, the AFR would be say 14:1. When I would open the primary throttles up (not yet opening the secondaries and not low enough vacuum to open the PV), and raise the rpm to say 4500, the AFR would go rich 12:1. Basically, as the air flowed more, the tune got richer even just on the primary high speed circuit. Would increasing the high speed air bleed in that situation even out the richness?

Thanks for any help you can give. There really is not much info on air bleeds, but I am at a point where out of the box jet only tuning is not getting rid of the quirks, so I am hoping air bleeds will get me there.
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Old 05-11-2009, 10:15 AM   #2
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Grab some coffee

http://www.innovatemotorsports.com/f...3&d=1177457666

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I try to "jist ya" on it

An air bleed is going to change the all you mentioned,AND the curve of your fuel. SO the HSAB's (high speed air bleeds), you will deal with only if your fuel curve isn't flat. Open the up to lean it out big end or vise versa. The more air you cram through the boost (stronger signal, more fuel), the more fuel the booster will flow. If you open the air bleeds, some of that air will bypass and go around the booster, and not flow the same flow.
Now it takes a certain amount of air to get a certain amount of fuel pulled on any engine. In theory a carb will flow the same amount of fuel at a given vacuum. So if your circuits are starting just a little late and you want to start them a little earlier, you can add more signal (air) to the booster by closing and air bleed. This will get you moving more air earlier, getting you to your point sooner, if this didn't all just confuse you more. Once you wrap your head around it, it is fairly simple. took me a while.

Sounds to me like you jet are a bit rich, and you LS air bleeds could go a bit smaller. But start at WOT first to get your jetting and curve down. Once you get that down, you can check to see if the jets are too big. If they are, jet down to where it likes it and open the PVCR's.

There are other tricks also. Like raising you floats level one flat to start you mains a little sooner. that will make it easier to pull the fuel over its wall. You can up the idle speed up front, which will start the mains sooner. (close the rear to keep the idle speed to same) If diesels like you already saw, you won't be able to do this.
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Old 05-11-2009, 10:38 AM   #3
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Yeh, I have to run close to a square idle throttle position front and back to prevent dieseling. My best idle speed is about 900rpm. When it is warm, having the front throttle opened that much makes for dieseling issues. I also have very little idle response from the idle screws when the front is open more than the rear.

I definitely have a non flat air fuel curve. That is why I dumped the street double pumper. Basically with the previous double pumper, when if I set the cruise or mid range rpm AFR for high 13s, when I opened the throttle and gave the engine some air flow it would go rich. If I set the jetting so that WOT was about right high 12s, it would run really lean at cruise. This was much less of an issue when the temperature was 40-60* but now that it is 95+ the air fuel curve problem is really apparent. That is the big reason why I threw my hands up and got the Holley HP .

I agree that currently the HP is jetted a little rich. I will know for sure how rich on my first test drive, but before I start swapping out jets, I was trying to get a grasp on when to change jets and when to change HSAB. So, if your air fuel curve goes richer as the engine speed increases, then larger/leaner HSAB will help flatten it out?

Any idea at what point the carb changes from the transfer slot circuit to the main jetting? Is is at 10% or 25% throttle opening? That information would help me differentiate between the AFR of the transfer slot and the AFR or the primary jetting.

Why would you want a richer LSAB in the secondaries? I totally understand why you want richer jetting in the secondaries, but why have a richer idle circuit on the secondaries? I am having a hard time understanding why the factory tune had a much richer LSAB in the secondaries.

Again, thanks for getting me headed in the right direction and I will start reading your links.
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Old 05-11-2009, 12:43 PM   #4
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I have been doing a bunch of reading this morning. I am going to throw out another question on top of the ones above.

At what point or why would you want to adjust the idle feed restriction vs. LSAB vs. idle screw? I can assume that the idle screw only affects the actual idle AFR. Once the throttles are open enough for the T-Slot to function, the idle screw is bypassed. At that point the LSAB and IFR are affecting the amount of fuel going out the T-Slot. Why would modify the LSAB vs. the IFR? I did not want to spend the money on the Holley Ultra HP, but have considered getting aftermarket billet metering block from ProForm so that I can have adjustable IFRs...

After reading a bunch of articles about tunes from ProFrom and other performance aftermarket carb places, it appears that most of them run the same LSAB on the primaries and secondaries, or at least within a few sizes of each other. I still am not sure why Holley sent out my 750HP with LSAB 71 in the primary and 20 something in the secondary...
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Old 05-11-2009, 01:42 PM   #5
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Let's go back to theory some here, it might help you understand more what needs to be changed and why.

Essentially the way the carb works is air flows through the barrels and booster, speeds up causing a pressure drop(Yeah, you know this, bear with me). So we now have higher atmospheric pressure and lower booster pressure generating flow. But it generates flow through 2 separate systems that will merge into 1.

From the fuel side the atm pressure pushes fuel through the jet and it fills up the main circuit fuel feed well, which is basically a tube drilled in the metering block(that's what those plugs on the blocks are plugging, where they drilled it). Fluid seeks it's own level so the main fuel well will try to keep the fuel level even with the fuel level in the bowl. As the booster makes signal and draws the fuel out of the main well the level gets reduced, the job of the jet is to essentially control how quickly the fuel can flow back into the main well to equalize the levels.

Now, at the same time you have the high speed air bleeds doing something similar(think of it as an air jet for this system). The higher atm pressure pushes air into the main air bleed well through the bleed itself, which is parallel to the main fuel well. This is where the circuits now merge. The main fuel well and main air well have channels cross drilled between them, so that the air from the main air well can "bleed" into the main fuel well. In the case of the Holley HP series there are 3 of these emulsion holes at 3 different height levels drilled across from the main air to the main fuel well, plus there's always a tiny one(not counted as one of the circuits) drilled across above the fuel level to help flow start.

This serves several purposes. First, what may already be obvious is that as the fuel level is drawn down in the main fuel well it will start to uncover these emulsion circuits, drawing more air across and into the main fuel well. This is how carburetors prevent the vacuum signal from being too absurdly strong and over fueling the engine at higher rpm. This is also where you change the CURVE of the fuel. By having larger or smaller emulsion bleeds across the 2 systems at differing heights, you can control how much air is bled into the main system at differing rpm and throttle position. At WOT typically the fuel level in the main fuel well is at rock bottom and all the emulsion circuits are uncovered and bleeding air in.

The other thing it does is as you have the air flowing into the main fuel well it generates velocity(as it flows through the main air bleed, down the well, across the emulsion circuit and up the fuel well out to the booster) and lower pressure above the fuel which helps to reduce the surface tension of the liquid fuel and increase volatility. This then causes the fuel to start to break up into very small droplets and generate a froth which is easier to lift out of the main fuel well. This is why modern multi circuit carburetors are more sensitive to booster signal, it's easier to lift fuel out of them. Also by frothing, or emulsifying, the fuel it is more easily and readily atomized and vaporized, helping to make better power.

Now, the things to keep in mind. Like you already realized the air bleeds reduce signal or increase signal to the circuit. This is done by essentially being a controlled vacuum leak into the main fuel metering system. Air is easier to move than heavy liquid fuel, so the larger the air bleed the more air leaks through the system to the booster and the less fuel. This means you need a stronger booster signal to lift fuel out of the circuit. That means that larger high speed air bleeds will cause the carb to need a higher engine rpm before the booster "comes on" and a smaller air bleed will cause the booster to come on sooner. Also, by adding air into the system you lean it out(or taking air from the system richens it up). But, one thing to keep in mind is don't go TOO large with air bleeds from what the system was originally designed with, there can be a phenomina where there's so much airflow in the system that it creates such a high airflow velocity through the fuel circuit that it rips and carries large volumes of fuel with it, such that a larger air bleed can actually cause the system to run RICHER. If you notice that happen, STOP! You've gone too far and need to back up a ways.

On the idle side it's a bit different. Fuel that enters the main fuel well also goes into the main idle feed well. The vacuum from the intake generates signal in this case, and the pressure difference pushes the fuel up the main idle well and through a restrictor(either at the top or the bottom of the idle well) which is basically a jet for the idle system. At the top of the idle well it "spills over" and down to the idle adjuster screws and transfer slots. At the top of the idle wellw here it spills over is where air from the idle air bleed is introduced, breaking up the fuel and frothing it. Since the intake vacuum has to lift the fuel up the main idle well, this is where the idle air bleed changes system signal strength. A larger air bleed means more of the intake vacuum is "bled off" as the air tries to neutralize the vacuum, meaning you need more vacuum strength to lift fuel out of the idle well. And obviously a smaller idle air bleed makes for a stronger signal to lift fuel.

So, a larger idle air bleed will reduce the strength of the signal drawing fuel in and lean out the idle mix, and a smaller one will increase fuel draw and richen it.

So, now back to tuning.

First you need to get the idle circuit right. The idle mixture screws should be turned out somewhere between 5/8 of a turn and 1 3/4 of a turn. Anything outside that range might mean that a change in the idle feed restrictor or idle air bleed is needed, as it will indicate a rich or lean condition in the idle circuit which will effect the AFR from the transfer slot. If the idle system is out of whack then the car won't idle right, and will run like crap at throttle tip in and low rpm part throttle. Also you only want around .020 of the transfer slot uncovered. If things are way out of whack you need to go back to the base calibration, reset everything and start over, otherwise you may just end up chasing problems that you created and getting nowhere.

For WOT AFR start with jetting and if you have a wideband O2 on the dyno, USE IT!! See where it's rich or lean. Depending on how the main circuit is designed you may be able to change things around with an air bleed, but generally only if the AFR is out of whack at lower rpm when the boosters start to come on. If it's rich or lean at lower rpm and WOT then an air bleed can fix it, but like CPR said, it's not actually gonna change the fuel curve itself so much, that's part of the main emulsion system itself. If it's fine throughout but then leans out or richens up at the mid or top, that's gonna have to be changed in the emulsion system in the metering block.

Anyway, this is all more of a general info on carbs, to give you an idea of how and why they work and how and why jets and air bleeds change certain things.
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Old 05-11-2009, 01:48 PM   #6
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Good post, 67mustang302.

Yes, I am using a dual sensor LM-2 and a vacuum gauge to tune with.

Out of the box, the idle screws were at about 3/4 for the primaries and 1/2 for the secondaries. The primary throttle plate, once idle speed was set to 900rpm, was uncovering the t-slot, but not by more than .030. I knew from experience that this was going to cause me dieseling, which it did once the car warmed up. I turned the secondary idle set screw a 1/4 turn open and reduced the primary idle until speed was again at 900rpm. This left very little of the t-slot open on the front side and none open on the secondaries. The idle screws were set at a tick less than 3/4 turn on the primaries and less than a 1/2 turn on the secondaries. That is when I decided to change the secondary LSAB from a 20 something to a 45. By doing that, a good AFR at idle was achieved with a tick over 1/2 turn. Dieseling in this setup is minimal, but slightly more secondary throttle and slightly less primary throttle at idle should knock that completely out based on my experience from the other carb. I can assume that raising the secondary LSAB to 71 like the primaries are would put me at a comparably idle screw setting at a tick under 3/4 turn. I do not want to do this go from 20 something to 71, without knowing why Holley had the secondary LSAB so low (rich). Shouldn't the primary and secondary LSAB be close to the same?

So, I think I have a grasp on the HSAB vs. jet tuning, but the low speed idle stuff still has be scratching my head some. Am I correct in assuming that at 1200-1500 rpm (cracked throttle), the carb is running mostly off the T-slot? If that is the case, then am I also correct in assuming that you can set the AFR at the level with the LSAB? If that is the case, then I could set the LSAB and then set the idle screw for the idle AFR since it appears that both affect idle AFR, but only the LSAB affects the T-Slot AFR.
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Old 05-11-2009, 02:01 PM   #7
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Ok, I just read your last post. Are you SURE the LSAB in the secondaries is a 20?! That's WAY too small unless they're running a pinhole for an IFR! That's even small for a HSAB.

As far as idle and transfer slots, as long as there's enough intake vacuum to pull fuel out of the main idle well, then the idle and transfer slots will be putting fuel in the intake. Exactly where that is depends on the IFR and LSAB sizing, since they work together to determine how much vacuum is needed to draw X amount of fuel. Typically when driving around at low rpm part throttle cruise, most of the fuel is coming from the idle system, mainly via the transfer slot. The boosters start to come on around 2,000rpm(varies from carb to engine to tune etc). The higher the rpm at cruise the more booster signal and the more fuel is coming from the booster rather than the transfer slot.

As far as IFR vs LSAB....The IFR is basically the jet for the idle system. Smaller IFR means a leaner mix out of both the idle mix and transfer slot, and bigger IFR is richer. Larger is smaller IFR will also change how sensitive the idle mix screws become. The LSAB when it's larger means the idle circuit is leaner but also less sensitive, and smaller means richer and more sensitive.

Basically your idle problems or part throttle cruise at 2,500rpm and lower you want to be looking at the idle circuit.
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Old 05-11-2009, 02:05 PM   #8
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Also keep in mind that on the HP with a 3 circuit metering(non adjustable) the higher rpm at WOT the more emulsion is uncovered, so it's going to respond more at higher rpm to a HSAB change than at lower. IE, a larger HSAB will lean it out everywhere, but will lean it out more up top than at the bottom of the rpm, with kind of an average in the mid range. Like I said it depends on the emulsion design of the carb in question, the number and size of the emulsion circuits is going to determine what the fuel curve looks like and how the curve changes with air bleed adjustments.
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Old 05-11-2009, 02:21 PM   #9
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Take a look at my previous post...I was adding a few things in edit while you were typing your response. Sorry...

I wrote down all the exact original tune info on some paper.

The stock air bleed were:
Primary (LSAB/HSAB) 71/25
Secondary (LSAB/HSAB) 28/36
Primary Jet was 75
Secondary Jet was 80

Strange huh?

That makes sense about the emulsion orfices and how the fuel curve reacts to changes in the HSAB.
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Old 05-11-2009, 05:35 PM   #10
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Hmm, they did some weird crap with those air bleeds. My Quick Fuel was 70/36 for the low and high speed bleeds on both sides(out of the box anyway). I wouldn't be afraid to open the 2ndary LSAB to a 70 or 71 PROVIDED the IFR's are the same size on the 2nd as they are on the primary. If you have a pinhole gauge pull the blocks off and measure the IFR sizes. They may have run a much smaller IFR on the secondaries. The fact that the 2nd idle mix screws are farther in indicates that the IFR's are probably the same or close to the same size, and it's pulling more fuel(the smaller bleed is increasing vacuum strength to the idle feed well) so you have to dial the idle mix in.

How is your 2nd linkage set, is it 1:1 or a 40% or 60%? I ask because if it's 1:1 then part throttle cruise will be off both the primaries and secondaries, so you have 2 sets of transfer slots to make sure the AFR is correct on. If it's a progressive like 40 or 60% then the 2ndary transfer slots won't matter quite as much, since you'll only be using the 2nd at either idle or WOT, so the AFR can be off a tad there as long as you can correct it at idle.

As far as the low speed stuff, basically the fuel mix from the LSAB and the IFR drop down and feed both the transfer slot AND the idle mix screws, but at idle the transfer slot isn't but hardly uncovered, so very little fuel gets through there. The idle mix screws take the idle system mixture and then controls the ratio of that mixture to the air being drawn into the engine at idle. IE, the idle system has some X:1 AFR and you then dial that AFR further up or down when you add that fuel mix to the air the engine is drawing in at idle by turning the screws. But at part throttle low rpm cruise when the transfer slots are uncovered the AFR will be determined by the AFR of the idle mix combined with the volume of air the engine is drawing in at cruise, so the only way to change the AFR when running off the transfer slots is to change the LSAB and/or IFR. If you're reasonably close then the LSAB should get it dialed at part throttle, you'll just have to reset the idle mix screws.

Basically the IFR and LSAB work together to create a preset AFR in the mixture inside the idle circuit, but when it's released into the manifold it mixes with more air and leans out. The idle mix screws allow you to control what volume of that premix is released when at idle, allowing you to go rich/lean at idle. Also the LSAB will work like a typical air bleed by increasing or decreasing signal strength to the circuit, so too large a LSAB for instance may cause the transfer slot to hesitate a moment before it starts to deliver fuel at throttle tip in.

You need to find out what size your IFR's are though.
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Old 05-11-2009, 05:35 PM
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