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I recently installed a Street Avenger on my 383 stroker. It seems to work well, but everyone, and I mean just about everyone, tells me to ditch it and put a double pumper on it. Can anyone explain why this would be such an improvement? Why would a double pumper carb be so much better than a vacuum secondary? Specifically on this type of motor.

Some of y'all probably know the build by now with the 725 million questions i've asked, but its a basic street 383. 10.6:1 compression. promaxx aluminum 200 heads. Dual plane intake. Crower roller cam 238/240 at .050 and mid .500 lift. Running 1.5 rockers on both but going to order some 1.6 for the intake side on it.
 

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The reason why many say to do that is because most of the time those folks really know nothing about carburetors and how they work and function and they don't have a real clue on fine tuning one. 75 percent of the folks I have came across over the last 13 years that I have been learning these things inside and out and experienced Holley and Quick fuel brand of carbs from vacuum secondary and double pumper alike on many different engine combos.

I have done a lot of research on them and have a lot of experiences with both styles and for the lack of a better term most of those folks who say to put a double pumper on your motor really don't have a clue what they are even talking about. Most of that crowd are the ones that know how to change jets and turn screws and that is it. They don't really take the time to learn the way each circuit works and how a carb works in relation to the build and what carb size will work well and the different styles there is. Carb tuning is not as easy as turning a few screws and changing out jets but a very unique process that takes time to learn to do especially in performance builds to where it gets a bit tricky with bigger cams and other things like high flowing heads and intake design being used and many other factors such as transmission type and the vehicle weight and how fast it gets up to rpm will play a factor in the whole package of things.

The biggest and main difference between a double pumper and a vacuum secondary wise is hitting it wide open throttle very fast or more then half throttle bursts up to 3/4 throttle openings. In an everyday driver and just doing normal driving, one carb will be no different then the other. A double pumper won't open up the secondary until around 40 percent of throttle opening. The vacuum secondary will not open up the secondary side until enough air velocity goes through the primary passenger side barrel in the front and it also gets air from the back barrel as well and the color of the spring being used will also play a factor in when the openings will open up.

Also a difference is the size of the vacuum secondary carb being used and on the cubic size of the build and how much air flow the engine can take in along with the incoming air velocity signal will play a factor in things as well. At a dragstrip the double pumper will be better in making the secondary side open up wide open throttle and hitting things fast and hard vs a vacuum secondary carb in most cases but sometimes the double pumper can be outdone by a vacuum secondary if tuned right and depending on the vehicle and the engine combo.

Several factors can affect a double pumper and make it not useful on certain vehicles. A vacuum secondary carburetor is way more forgiving on the size and also the opening of the secondary side and function of it vs a double pumper carb. Basically a double pumper carburetor needs to be on a vehicle that is fast reviving and also that catches rpm very fast and also does not take much to get the vehicle up to speed quickly and something that is not over heavy.

A double pumper on a vehicle that takes time to gain rpm and also is on the heavy side of things will be a dog and most likely bog really bad and be a bear to tune the secondary side of things an in some instances a nightmare to deal with as it will just double down and just give to big of a input of air velocity going in and cause the engine to just run like crap until it gets enough time for the engine to pick up the rpms needed to be able to handle the load being put on the build. A vacuum secondary will work better in that type of a case as the vacuum secondary side will only open up to what the engine needs and will open up at a better rate if the correct spring is used and a very close size carb is used as well on the build as the secondary side of a vacuum secondary can be used across a lot broader range of builds vs a double pumper.

You can use a vacuum secondary carb on a heavy vehicle and the secondary side will be a lot more slower to kick in and a little bit longer opening up wise vs a double pumper and will allow the engine more time to catch up rpm wise to get into the rpm range it needs to get running good. The thing though is that the transmission and also the rear gears along with the camshaft being used also need to be matched well with the vehicle too as that will also play a factor into things. Things front to back need to be matched before just throwing a carburetor on and getting it tuned right.

There are some myths that go with a double pumper vs a vacuum secondary as well such as fuel mileage. The front of a double pumper is no different then a vacuum secondary carb and both will get equal mileage in normal steady cruising and if you keep your foot out of the secondaries and don't keep stomping on things and getting past 40 percent of throttle travel. If you keep smashing the pedal then yes a double pumper will get less mileage because of the secondary pump shot. The reason why many say you will get less mileage is because they don't know the advance tuning things of a carb like the idle circuit.

On the older style Holley carbs the double pumper carbs are setup with a lot richer of an idle circuit compared to a Holley vacuum secondary of the 600 and 650 cfm range size. The 750 cfm double pumpers are a 4 corner idle setup vs the other 600 and 650 which are a 2 corner idle on the old 4776 and 4777. The double pumper carbs are calibrated for engines setup with a lot more camshaft duration and bigger flowing heads and more overlap.

The vacuum secondary Holley carbs like the 670 street avenger carbs and the 80457 and 1850 600 cfm vacuum secondary carburetors are calibrated way leaner and more towards milder engines and smaller camshaft duration and for stock to mild built motors. Now when you get into the Quick fuel carburetors and the others like them then you get into a whole different ballgame as most of them are all calibrated on the way richer side vs some of the older Holley classic carburetors that you can find everywhere out there such as the few I mentioned above.

A double pumper carb is best used on light vehicle that has a medium range to more steeper gearing and also using a manual transmission or if using an automatic transmission using a stall convertor more then stock and that is matched close to the camshaft being used. On the Holley website it says you can't use a double pumper on an automatic transmission unless using at least a 3000 plus stall converter but that is most likely them being on the safer side of things, but I have used one on my s10 build that I had years ago that I sold and also my current build and ran it just fine as I used a correct cfm range for my truck and with it being light and with a good rear end gearing matched up to my turbo 350 and my truck picking up rpm's very fast it worked out just fine with some basic tuning.

I for the most part like the double pumper more vs a vacuum secondary but the only benefit I really ever got was just a quick opening on the secondary side when getting into it and romping on the throttle but the overall difference is not that much different for everyday cruising and I don't race at the track so regardless of what I run I really don't gain or loose anything on all around performance wise. Just like everything they both have there pros and cons depending on what and how its going to be used and what your looking for. Sometimes one can be better and sometimes one can be worse and hurt performance but it just depends on the application.

There is a lot more I can type here but that if just a few basics of it. At a track is where the both of them can be a bit tricky as to what is best and sometimes a vacuum secondary according to some of the Holley tuning books can give better steady times at the track vs a double pumper depending on certain conditions.
 

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I have a dart shp 377 currently but changing that soon. Its got a 4.155 bore and 3.48 stroke and 10:1 compression. I run Dart pro 1 200cc platinum heads with an Edelbrock Rpm air gap intake manifold. The heads were all ported and blended in the bowls by my Father who has built many performance builds from all three big auto makers for over forty plus years and knows his stuff. Camshaft is a custom hydraulic roller grind from Mike Jones cams which specs at 268/272 220/[email protected] 510/510 lift with a 114 lsa and I get around 18 inches of vacuum in park.

I have a turbo 350 with a set of 3.42 rear gears and a Detroit tru trac posi unit in the rear end on my chevy s10 and I have ran everything from a Holley 650 double pumper and Quick fuel brand as well and also ran a Holley HP Double pumper on it before as well and I had ran all of them great.

I am currently running a basic Holley 600 vacuum secondary 1850 which in stock trim will not run on my build what so ever but I have changed and customized a few things and recalibrated it on the idle circuit and changed the size of the air bleeds to what makes it run fine on my build. It's not all fancy like a four corner idle but I am running a milder cam and a four corner idle is not needed for it. I for myself like to be conservative on my carb size and like the 650 ish range to get a stronger signal and a crisper response on the low and mid range rpm vs a 750 which I have used before and was really nice and don't get me wrong it was a very responsive build with a slightly bigger cam then what I listed above and was a really bad beast when getting on it but I like the better response on the bottom end and mid range while cruising.

My custom build carb was a regular Holley 80457 600 cfm build but I changed a lot of things on it. Money wise it would have been cheaper for someone who is starting out from scratch to just buy a whole complete new carb but I had spare parts laying around and only had to invest a few hundred bucks and made it as good as a 600 dollar carb.

I took the main body and pulled out the straight leg boosters and installed downleg boosters that are stepped for a stronger signal to the boosters and the step on the boosters help with fuel atomization and I then installed a bigger 1 11/16 billet base plate which makes it a 650cfm size carb and then I installed a set of Quick fuel billet metering blocks and then swapped out the side hung fuel bowls for a set of center hung fuel bowls. I then put on a Quick fuel adjustable pod to adjust the secondary side to get the best opening and setting I can and it works way better then the stock Holley quick change or stock cover pod as you can't adjust anything except change out the spring vs on a Quick fuel pod it allows you to adjust the sensitivity rate and opening of the secondary side on whatever spring you use and allows you to adjust it for the best you can get for what ever build you have.

My Holley or what ever vacuum secondary carb runs very strong on my build once setup and dialed in correctly. The Holley street avenger carbs are lacking on some things and are basically a sleeping giant and just need to do a few things and they can be woken up and run like a champ if one takes time to do so. Just a few upgrades can make a world of difference on them. They for my opinion are over priced compared to other carb brands but sinnce Holley bought Quick fuel out they have gone cheap on there other brand of carbs and many no longer have the ability to change the idle feed restrictors and power valve channel restrictors out in the metering blocks and have taken out features like down leg boosters on the more cheaper carbs like the Slayer series and the Brawler carbs.

Depends on what Quick fuel brand you get. When Quick fuel was in business for themselves there carbs were the bomb but since Holley bought them, they are not as tuneable and are cheaper made but more expensive vs the older days of when Quick Fuel was first came out. Your street avenger cam be made to run really well and don't know what size your running but a few upgrades can make it wake up. It has been a night and day difference on those when you know what to do to wake them up. They are good carbs but its just said there are better options for carbs in the same price range as they cost. What exact one do you have?
 

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What Eric said.

In short the mechanical secondary comes on fast and often excessively for the immediate air demand of the engine. This both results in a drop in velocity through all the induction system which causes a dip in fuel metering at the boost venturies. So the mixture in the manifold plumbing slightly stalls while the metering goes lean, the pumper on the secondary is a CYA action to minimize the resultant stumble.

With a vacuum secondary whether that is the Holley diaphragm or the Carter/Edelbrock and the Q-Jet mass flow door; the secondary flow is allowed in measure to the engine’s immediate air flow demand. As a result there isn’t this flow stall in the system from air horn to the intake valves and the measured opening to mass demand on the secondary side allows main metering to come on without a lean drop in AFR so there is no need for a secondary accelerating pump.

If your racing the potential issue with a vacuum secondary of whatever type of flow sensitive device is there then there also is a drop in flow as some teeny, tiny bit of energy is pulled off the flow volume to operate the mechanism. Ya know it’s physics there are no free lunches. This is seen as a slight deviation from the flow device being absolutely fully open as a result the engine at WOT is a small fraction of a pressure inch short of maximum absolute manifold pressure the engine can otherwise achieve.

So if you're a contender on the local or national or international racing stage where everything else about engine and vehicle is known to be maximized, then and that still is an “if-then” a mechanical secondary with an accelerating circuit is something to worry about. For posers it’s just a decoration on their suit, or in this case engine. They probably can’t even explain why it’s there beyond the really fast pro racers use ‘em, therefore, so do I.

Bogie
 

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Ok I saw that you have the Holley 770 Street avenger and its a lot more calibrated for wilder builds vs the 670 street avenger. I don't know what all your expecting out of your build but if you go to a double pumper the only thing you will see different is when getting on it hard and hitting the throttle hard. Just because you have a manual transmission does not mean the double pumper will work well on your build right off the bat and gain you a lot more for performance wise. Like I mentioned in my previous posts I don't gain really much more when going to a double pumper vs a vacuum secondary when all things were tuned to there best.

The street avengers are nice carbs and with a couple upgrades they can come alive even more and operate a lot more better then what they are in stock trim. Even with a Brawler carb you might not gain that much except have the ability to change the air bleeds and have a four corner idle but I don't know if your street avenger is the older two corner idle one, or the newer four corner idle ones they changed them out to several years back. With a few things you can have your carb to do just about as well as a Brawler equivalent of a vacuum secondary type with just a little cash and some time and a few tools and customizing a few things.

On a lot of the newer Quick Fuel carbs since Holley has bought them out they took away the ability to change out the idle feed restrictors in the metering blocks along with the power valve channel restrictors. The Quick Fuel style metering blocks are a bit better quality vs a Holley metering block of the cast variety. I bought a Quick Fuel Brawler double pumper last year and got the one wit the billet blocks thinking it would have all the changeable areas on the blocks but nope, the blocks had pressed in restrictors and the way they was designed I could not fix the big .070 power valve channel restrictor that was on a 650 cfm rated carb and the .070 is more for a 850 to 950 cfm range carburetor and not a 650 and would have been so pig rich at wide open throttle it would have been horrible.

I eventually sold it as I needed cash for some other things but I don't know about the cast series but stay away from the billet series if you decide to get one. Bottom line if you want a carb to be like the brawler you can get yours very close for about a 1/4 of the cost and keep your current carb and do a few upgrades on it and wake it up and trust me you would get more performance if you took the time to fine tune your current street avenger and try to get the best amount of it and it should perform quite well for your engine and be satisfactory on it.
 

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Get rid of the quick change secondary pod and buy you a Quick fuel adjustable pod and then buy a set of springs that has all the colors in it and you can try out the lightest spring first and go at it to adjust it using the screw and if you can't get rid of the bog if you have one then go one spring up and then go back again adjusting the screw a 1/4 of a turn at a time until your bog is gone if you have one.

I myself tried the white spring which is the lightest and it would bog on my s10 and I used the quick change pod and I could not get what I wanted from the secondary side and was lackluster at best. I tried the four lightest springs and could not get things dialed in like I wanted. White was too fast, both yellows would work in some instances just fine and other instances be to fast by a hair. Went to a purple spring and it was good but at times felt like it would not open up fast enough to what I wanted.

I then took the pod and drilled the brass restrictor that is inside the Holley pod a little bit bigger to allow more air in to allow the secondaries to open up a little bit quicker and get more response and it worked but I went a little to much though as it was just a trial thing and picked it up from an article in Hotrod magazine. I already had a Quick Fuel topper which I had on standby anyways just in case. I then drilled out the brass restrictor from the vacuum pod and then stuck on an adjustable Quick fuel style top cover which I got from a guy from ebay who sells Holley parts in bulk and stuff and the Quick fuel pods don't have the brass restrictor or sometimes if you have heard the term a check ball in them since they have the screw which you can use to adjust the rate of the incoming air signal coming into the pod.

Holley on there street avenger carbs and all vacuum secondary carbs they have either a check ball which has not been used in many years and anything made in the last ten years or so have a brass restrictor in the bottom part of the pod and it has a .040 hole in it which is to allow and restrict the amount of air coming into the pod. Its ok on a stock build but on a performance build the Quick fuel pod really makes a big difference once dialed in.

If you just buy the top adjustable pod part then you have to drill out the brass restrictor in the bottom part of the pod or you won't get any real difference in anything because even with the adjustable screw you still have the brass restrictor in the bottom and it will keep the adjustable top from being able to do its job. With the Quick fuel topper I can now adjust things even more then just by the spring rate but also by the sensitivity and amount of the incoming air is. The adjustable pod with the lightest spring your able to use will make it wake up quite a bit since you can adjust things. Woke my carb up a pretty good bit.
 

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As pointed out here the Quick Fuel carburetor I purchased a few years ago is a better built, more easily tunable, less costly than what is offered today. The older one has brass tunable everything. The new one only the idle bleeds everything else had to be threaded and/or drilled and requires restrictions and/or plugs for “too big or unnecessary ports”. Also none of them has lower IFR threaded. But in my opinion the Quick Fuel is still the best option out there even though it is NOT USABLE RIGHT OUT OF THE BOX.
 

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Discussion Starter · #8 · (Edited)
Wow, thank y’all for the responses. This site absolutely blows away any other forum I’ve ever been on, car related or not, in the amount of knowledge people have.

This is a brand new street avenger. Four corner idle adjustment. I’ll look into converting the secondary pod to an adjustable one.

Eric I read on one post where you have modified these carbs to accept adjustable Air bleeds. I need to look back and find that. Or, have you ever made a video of what you do? Also, have you done or heard of adding a secondary air bleed tube?
 

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Bcjones, yes I have drilled out the air bleeds before in the Holley main bodies and I bought extra long taps to do the primary side of the bleeds. I have six inch long taps for the primary side that I use to thread the holes for the brass set screws. I got mine from mcmaster.com. And yes I have heard of the brass tube modification on the main body as well but I have not done that. On the 650 and smaller vacuum secondary Holley carbs I have had the secondaries open up without any issue and be pretty fast with no tube in them. Now I have never used a 750 aka 770 vacuum secondary before and on the original Holley vacuum secondary carburetors in the early days when they made them until the late 70's, they had the brass tube in them with the angle cut to help open up the secondary side better and improve the signal and quicker opening.

It seems by the very early 80's they quit putting them in as I have an original Holley 80457 600 cfm carburetor that came out in the early 80's and it had no tube in it and the early model 3310 750 aka Holley 780 I do believe were the first and only few that had the tubes in them and Holley quit putting them in after that. Quick Fuel carbs so far even after Holley bought them still has the brass tubes put in regardless of the cfm size of the carb to help improve that signal. For me I have not had any difference on the secondary side as far as response goes on the 650 or 600 rated carbs goes in my experience with or without the brass tube installed for overall performance.

I know its not hard but for me to get a tube and go through the hassle to install that in a Holley main body when it works ok without it is not worth the work and time for that when I get everything I need with or without it. Its sad because the 750 or 770 Holley street avenger which both are the same main body just with a different number stamped on the tower and a different marked rating for commercial purposes and selling purposes wise, they don't seem to work as well on the secondary side because of the bigger barrel and many folks from what I have read could not ever get the secondary side to open up all the way even with the lightest spring.

But I don't know the builds that the people had and also if there engines could use all that air flow that the carb could allow and I don't know how they tried to tune it as well. Also they had the older pods where they had a check ball and the check ball would help the secondary side to not open up to quickly and also not snap back shut to fast and many would take the check ball out. In the pod with the check ball it had a little dimple made into the spot where the ball would seat to allow it to allow some air go by in order to activate things.

The problem many had is when they took the check ball out it would allow to much air to quickly and it would make it bog and they would then use a stiffer spring more then usual to get rid of the bog but because going to a stiffer spring they would not get it to open up all the way. A trick many used was take a phillips screwdriver and they would take the pod and strike it with the screwdriver inside making more dimples and thus allow more air flow pass the check ball.

Many things were done over the years to help on the 750 cfm vacuum secondary to help with things and Holley taking out that brass tube most likely was a cost saving thing as in the 80's many of there carbs were made with plastic pieces for the manual choke housings and parts and the accelerator pump arms were plastic as well and the top of the vacuum housing was plastic as well. Eventually then went back to all metal parts and ditched the steel check ball and went to the brass restrictor that they currently use on there Holley brand vacuum secondary carbs only.

The brass tube is really needed more so no the bigger vacuum secondary carbs vs the smaller ones as because the smaller ones have the smaller barrels they have a chance with the smaller bore and thus also creating more velocity much easier and a stronger signal many times don't need the brass tube but it certainly does not hurt anything. I have read how they do it and I have it down in my notes and stuff and have not attempted it yet but I am sure I could do it if I had the parts needed to do it with.

The air bleed mod is easy to do and I was successful the very first time I did it but I 90 percent of the time have never needed to change the air bleeds. I have had way more success drilling out the pressed in restrictors in the metering blocks and using the brass allen set screws and change them in order to get my idle circuit in the proper range on what I need. I also buy screw in boosters from allcarbs.com if I want to pop out the straight leg boosters and install the downleg boosters to help get a stronger signal to the boosters and some slight more air flow.

I don't know where I would get the brass tubing needed to install in the main body to do that mod and it seems it would not be hard but I myself would not really want to do it if I did not have to. Its just sad how Holley has cheaped out on things and the crap junk I have gotten over the last couple of years has been disappointing for me. I have gotten at least five carbs from them on there Holley brand that I had to pop out the boosters as I could wiggle them by my fingers back and forth over 1/16th of an inch from side to side. The cost of the carbs nowadays is getting really out of hand and is making it hard for folks to get what they need for a nice ride and quality control from all sectors across the board for aftermarket stuff is just sad compared to what used to be.

You can thank China outsource for that to a certain degree but also the companies are to blame as well for wanting to make more money for themselves then give a goop paying job to there employees and a good product to those who support them.
 

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For all the work that needs to be done on a new Holley or Quick Fuel one might be better off quality and cost wise to seek something used.

Definitely a Quick Fuel prior to Holley acquiring them is better. Just as a casual observation a Holley model for model costs even more with less features.
 

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Eric I read on one post where you have modified these carbs to accept adjustable Air bleeds. I need to look back and find that. Or, have you ever made a video of what you do? Also, have you done or heard of adding a secondary air bleed tube?
If you are referring to the vacuum secondary signal passage mod, where you press a slash cut brass tube into the diagonal vacuum secondary signal passage so that it sticks out 1/8" or so into the back corner of the passenger front primary venturi.....then this is absolutely THE very next thing you need to do to the carb, even before getting any kind of quickchange or adjustable cover.
Holley has quit putting this brass tube in the bigger vacuum secondary models(probably as a cost saving measure, who knows) but it really kills the secondary response. The tube sticking out with the 45° slash cut on the end facing downward turns it into a tiny booster to amp up the secondary signal.

i've seen one post about it claiming 4-5 tenths and 4 mph difference on a low 12 second car with a 770 Avenger..

Some info for you to research.
(5) Vacuum Secondary Diaphragm Signal Tube - racingfuelsystems (tapatalk.com)

There is some drilling and tapping involved, you have to source the brass tubing (hobby/craft stoke), step drilling the passage....but it isn't terribly hard. Not much different that the process of converting to screw-in bleeds

Absolutely needs to be done tho'.

Then get the Quick Fuel adjustable cover when you put it back together. I'd geuss probably want the Yellow spring in the secondary, it'll be too fast for your combination, which allows you to dial it back with the cover adjustment.

You'll likely be surprised how much better it'll feel.
 

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Discussion Starter · #12 ·
If you are referring to the vacuum secondary signal passage mod, where you press a slash cut brass tube into the diagonal vacuum secondary signal passage so that it sticks out 1/8" or so into the back corner of the passenger front primary venturi.....then this is absolutely THE very next thing you need to do to the carb, even before getting any kind of quickchange or adjustable cover.
Holley has quit putting this brass tube in the bigger vacuum secondary models(probably as a cost saving measure, who knows) but it really kills the secondary response. The tube sticking out with the 45° slash cut on the end facing downward turns it into a tiny booster to amp up the secondary signal.

i've seen one post about it claiming 4-5 tenths and 4 mph difference on a low 12 second car with a 770 Avenger..

Some info for you to research.
(5) Vacuum Secondary Diaphragm Signal Tube - racingfuelsystems (tapatalk.com)

There is some drilling and tapping involved, you have to source the brass tubing (hobby/craft stoke), step drilling the passage....but it isn't terribly hard. Not much different that the process of converting to screw-in bleeds

Absolutely needs to be done tho'.

Then get the Quick Fuel adjustable cover when you put it back together. I'd geuss probably want the Yellow spring in the secondary, it'll be too fast for your combination, which allows you to dial it back with the cover adjustment.

You'll likely be surprised how much better it'll feel.
I'm going to do it this weekend. Along with a couple other tweaks. Seems to be very worth the effort. I'll be honest and say I have never verified the opening of the secondarys. Im going to assume they have a least been partially opening, but I cant say I know they are fully opening. From what i've read, a lot of people have trouble with it, even to the extent that the lightest spring still doesn't fully open them.
 

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With all this carburetor knowledge I was wondering if there was any truth to the idea that a vacuum secondary was
generally used in cars with automatic transmissions because of how vacuum secondaries open more readily with the torque multiplication process that occurs in the torque converter. and double pumpers were more responsive to manual transmissions demands for instant torque ? Sorry don't mean to hi-jack the thread it seems there a lot of knowledge here to either confirm or dispel this myth! thanks in advance for all your carburetor knowledge gentlemen .
 

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That is a myth to a certain degree because of marketing and also people putting the wrong size on a particular build and also not getting the proper tune and also mismatched parts come into play as well. I have a turbo 350 in my chevy s10 with 3.42 rear gears and I once had a 2200 stall convertor and ran a double pumper just fine but it was a 650 cfm rated one and just with some minor tuning the pump shot on the primary side first then going to the secondary side I got it to run just as efficient as a vacuum secondary for the most part without any band aid type of a tune. I also have a shift kit in my transmission that I can manually shift it and keep it in first gear clear up into what ever rpm range I want before shifting into 2nd then into drive myself with my floor shifter.

The reason is my engine and truck can pick up the rpm's very fast and that is one thing a double pumper needs is an engine that can get into the rpm range very fast and not lug the engine and take to long for it to catch up during wide open throttle conditions or getting into the secondary side fast. Things have to be matched well in order to use one and even on a manual transmission if you have a set of rear gears that is really poorly matched to a cam that requires a way higher rpm range your going to really bod the thing down and when placing a double pumper that is too big for said build you run into it being not as forgiving on opening the secondary side of things as the signal to the carburetor will not be as strong and in response to air velocity and other factors and cause it to be a bear to tune wise without a band aid sort of fix but other factors can come into play.

Holley over the years and also many opinons on the double pumper vs vacuum secondary debate has become taken as truth without real world results and experiences and other factors can make one work or not work depending on more things. It all came down to what this person heard or that person heard and that information got passed around as facts vs actual truths over time and like an old wives tail is not always the truth in some conditions. Like all things they have there place and is more picky of the said conditions and the vehicle weight and transmission style along with things being matched up across the board from front to back vs just one particular part of the vehicle and the conditions its being driven in.

Take for instance you would not want to put a 750 double pumper on a bone stock 350 with a very small cam and poor flowing heads and economy rear gears along with an overdrive transmission as it would be a soggy dog when getting into it and just be a mess to tune with vs putting on a 750 vacuum secondary as the secondary side on a vacuum secondary carb would only open up to what the engine needed and that would be with a manual or an automatic. Its more forgiving on a manual vs an automatic because of the manual shifting of the transmission and it getting into the rpms quicker as you control the shifting point when you want it to shift and some other factors vs an automatic which unless you have a automatic/manual shift kit installed to where you can control to a point how high you can shift it yourself manually before you go into 2nd and then 3rd and overdrive if it has it can be a big difference in performance and response vs an automatic that only shifts on its own point just left in drive as it is just left in wide open throttle and with the secondary side just kept open up with out things picking up fast it can make it a dog vs a manual.

More to it then just saying its more for a manual vs an automatic transmission deal. It depends on how the said user will use there whole package of there ride and what do they want out of it and also what is the best for it overall. Matched parts can make or brake a tune on the carb and the vacuum secondary has a way more forgiving range vs a double pumper as with a dp carb there is only a certain range of how it functions to be able to be used in said application.
 

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My 502 BBC came with an 870 Street Avenger from the factory, the motor ran rich and the secondary's would not open, even with the lightest spring. Sometimes they would partially open. I went to a AED Holley 750 DP for street use, and I have no problems. Motor runs great.
 

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I had my truck out yesterday trying to tune out the bog I get from my secondary side opening up to fast. On my previous 350 I could use a white spring which is the lightest of them all and using a regular Holley quick change pod like the street avenger series comes with and I would not get a bog and it would open up all the way. I now with my current 377 build just wants to gulp my carb up and I have been having a heck of a time getting the thing to not want to open up the secondary side to fast. I put a quick fuel pod on and started out 1 1/2 turns which is a good starting point and I had to keep turning that thing in as it would just keep stuttering and would not completely bog but would hesitate for about a second or so until the boosters started to flow fuel.

I wish I had kept my old double pumper as even though I finally got my secondary stuff about 95 percent fixed and I had not more time to test yesterday as it was getting dark out, but I got rid of most of it and now is almost spot on but its been a way more pain for me on this build vs any other build I have ever done in my life time. I am going to save up some more cash and buy a replacement 650 double pumper main body or if I am lucky a used one off of ebay. I like the vacuum secondary carbs but on certain builds they just don't seem to do as well even though you dial them in and on the secondary side of things on mine anyways I have to keep having the secondary side to not open up to fast but at the same time it comes to a point I will never get the secondary side to open up all the way with the purple spring.

According to the chart it will not open up all the way till around 6000 rpm on a 400 cubic inch build and 6900 on a 350 build but will vary some. I hate it that I can't use a lightest spring to get my secondary side to fully open. Anything light my truck does not like as it picks up rpm's so fast and throws you through the back seat just hardly touching the throttle and its a very snappy build. The double pumper would just run hands down way better over the vacuum secondary when getting into it and stuff and you know it would just always open the way you like it but I never got into it that much to need it all the time but at least you know its there if you need to tromp on it especially when you need to pass somebody on the interstate.
 

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I called Holey about my problem and they told me to go with the 750 DP, they couldnt explain how two big companies GM & Holley fitted the 502 crate motor with the wrong carb???
I called AED and they also told me to use a 750 DP. They customize the Holley for your application.
 

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Suggest find an OLDER VERSION of Quick Fuel 750 that has all the adjustability that the newer Holley owned versions do not have. Your option vacuum secondary or not.

The adjustable vacuum secondary is a super feature along with the brass fitting adjustability that seems to have been eliminated on the Holley version of the Quick Fuel along with other “cheapening”
 

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Lots of good information in this thread!
I strongly recommend installing a wide band gauge for more exact carburetor tuning.
This have helped me a lot in getting my QFT double pumper on my car properly adjusted.
 

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Discussion Starter · #20 ·
I did the secondary “tube mod” and I could definitely tell the secondaries were opening better. I put a zip tie on the vacuum pod linkage and “measured” when the secondaries were opening. They would fully open but it would happen pretty late in the rpm range. I swapped the standard spring with the lightest one thinking there would be a bog on initially acceleration. Man, was I wrong. The thing lit the tires up from a 10 mph roll and didn’t stop spinning them all they way to the top of second gear! The secondaries are opening much sooner in the rpm range but not too soon to cause a bog. Feels like a different truck! I’m currently reading David Vizards Holley carb tuning book and it’s helped quite a bit understand what’s going on.
Thank y’all for the continued help. It’s amazing what these small changes will do!
 
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