q-jet info



q-jet info

Postby grumpyvette » January 31st, 2009, 11:17 am

got a quadrajet, this info should help, read thru the linked info

it helps a great deal to know what your working with , each carb has good and bad points, youll find links to parts and a great deal of info below, and working with and buying parts from people that SPECIALIZE in WORKING WITH these carbs is smart



http://www.smicarburetor.com/products/s ... oductID/76

https://www.smicarburetor.com/products/sfID1/28/sfID2/9

viewtopic.php?f=55&t=635

http://www.smicarburetor.com/

viewtopic.php?f=27&t=4714&p=18266&hilit=+drill+bits#p18266

http://www.cliffshighperformance.com/parts.html

http://quadrajetcarburetors.com/progr/c ... .JPG&idx=7

http://quadrajetparts.com/rochester-qua ... c7d9416ab4

http://www.carburetion.com/quadrajet.asp

http://quadrajetparts.com/rochester-qua ... p-557.html

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QUADRAJET

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http://www.hioutput.com/tech/qjetrod.html

http://www.circletrack.com/enginetech/c ... index.html

http://www.carburetor.ca/carbs/tech/Roc ... index.html

http://www.cliffshighperformance.com/te ... _ID_2.html



http://www.73-87.com/7387garage/drivetrain/myqjet.htm

http://www.docrebuild.com/dr-r-web/SPARK2.pdf

http://www.underhoodservice.com/Article ... etors.aspx

viewtopic.php?f=55&t=211

http://www.corvetteclub.org.uk/files/do ... a_qjet.pdf

http://www.stratagaz.com/Quadrajet/Quad ... 029202.htm

http://www.florida4x4.com/tech/quadrajet/index.php

http://arapaho.nsuok.edu/~leeaq/docs/QJ ... _Paper.doc

http://www.73-87.com/7387garage/drivetrain/myqjet.htm

http://www.highperformancepontiac.com/t ... _hose.html

http://www.carcraft.com/howto/57178/index.html

http://www.highperformancepontiac.com/t ... to_01.html

http://highperformancepontiac.automotiv ... -hose.html

http://www.buickpartsdirectory.com/carbs.htm

http://www.73-87.com/7387garage/drivetrain/qjetidle.htm

http://www.geocities.com/gtopercy/Pictu ... tTech.html

http://www.mako.com.au/buick/html/qjet_tips.htm
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: q-jet info

Postby g10van8772 » May 9th, 2009, 9:29 am

Well, in my travels I ran across this site.

http://www.vetteprojects.com/kstyer/quadrajet.htm
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Re: q-jet info

Postby grumpyvette » April 29th, 2010, 6:52 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: q-jet info

Postby grumpyvette » July 12th, 2010, 1:18 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: q-jet info

Postby grumpyvette » July 12th, 2010, 1:18 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Location: florida

Re: q-jet info

Postby grumpyvette » April 27th, 2011, 2:30 pm

This is written by Lars Grimsrud, you might find what you want to know here, most of it is correct.................

How to Tune a Q-Jet
(basic)


This tech paper will discuss basic set-up and tuning of QuadraJet carburetors for optimum street performance and drivability.

The procedure outlined here differs from other I have seen, and is based on my years of experience doing this work in the quickest, least painful, most economical way. It is recognized that other people will have different methods of doing things, and may disagree with specific methods and procedures that I use.


Overview
The Rochester QuadraJet, in its various forms and configurations, has been used by various GM Divisions for various applications since the mid sixties. The last passenger car version of the carburetor appeared as an ECM-controlled carburetor in 1981.

The Q-Jet is a highly versatile, tunable carburetor that will provide outstanding performance and reliability once set up correctly. This paper will discuss the tuning and setup, and will provide you with adequate data to make good decisions when jetting and adjusting the carburetor. This paper will not discuss basic rebuilding sequences, nor will I discuss operations involving machining operations and other severe alterations to the carburetor. There are many books on the market that deal with these subjects in depth. Rather, I will describe the various systems, their purpose, and a good tuning sequence to help you get each system and parameter set up correctly in the easiest way possible.

QuadraJet carburetors have three basic tuning variables, and these get people all confused: Primary Metering Jet, Primary Metering Rod, and Secondary Metering Rod. Attempting to cure problems by tuning the wrong variable results in lots of frustrations for tuners and car owners.

These systems at times overlap in their operation. Not only does each system need to be properly tuned, but its timing and “overlap” with other systems is critical to proper performance.

When tuning, we think of each of these variables as controlling a different operating range: The primary jet size determines the fuel mixture at Wide Open Throttle (WOT). The primary metering rod determines fuel mixture at cruise speed and determines responsiveness of the idle mixture screws and off-idle performance. The secondary rods are a high-rpm compliment to the primary side, and are used for final tweaking.

On a Q-Jet, we see that we can control the fuel mixture throughout the operating range. This is different from a Holley: A Holley has a given main jet size which meters fuel throughout the rpm range, including cruise. At WOT, the power valve unseats, and opens a fixed orifice, dumping a fixed amount of fuel in addition to the main jet. Crude, but simple and effective.

The Q-Jet meters fuel through the main jets. Metering rods, suspended from a power piston, plug off part of the area of the main jets by being inserted into the jets. These rods have a fat diameter and a skinny diameter: The number stamped into the side of every metering rod is the fat diameter indicated in thousands of an inch. This part of the rod is pulled into the main jet at cruise, at idle and at other high-vacuum operating conditions (light throttle). It produces a lean operating condition for good fuel economy and good throttle response. When engine vacuum is lost, indicating a high-power condition, the rods are pushed out of the jets by spring pressure, and only their skinny tips, or power tips, remain in the jets. This richens the fuel mixture up for peak power. All primary metering rods have the same power tip diameter (.026 ). This fact is crucial to remember when tuning: Primary metering rod sizes have no effect on WOT performance.


Identification

Q-Jets are identified by a number stamped into the Float Bowl casting on the driver s side of the car just above the secondary throttle linkage. If the carburetor is a Carter manufactured under license from Rochester, the number will be inside a round metal foil tag on the drive s side of the carburetor just above the primary throttle linkage.



Metering Area

WOT fuel mixture is controlled only by the main jet size. Performance at cruise and at idle is then controlled by the rods. We can establish each of these mixtures independently of the other by knowing and understanding the concept of Metering Area.

Jet and rod sizes are always referred to by their diameter in thousands of an inch. But fuel flow doesn t see diameters: The fuel sees the total metering area. So we must convert the diameter into a resulting area. We remember that the formula for area is pr2. Thus a jet with a diameter of .070 has a metering area of:

Radius = ½ diameter

Radius = .035

p.0352 = .00384

Thus, the metering area of a #70 jet is 3.84 thousands of a square inch.

But wait! There is a rod inserted into the jet, so we must subtract the area of the rod. Lets say we have a #40 rod in that #70 jet. The area of the rod is:

Radius = ½ diameter

Radius = .020

p.0202 = .00125

Thus, the area that a #40 rod plugs off is 1.25 thousands of a square inch.

The resulting metering area of the #70/#40 combination is thus 3.84 minus 1.25. The total metering area is 2.59 thousands of a square inch. This is the metering area of this rod/jet combination with the rod fully inserted in the jet. In other words, this is the metering area at cruise speed and at idle.

To see the metering area at WOT, we know that all rods have a .026 diameter power tip. So we run the same calculation for a .026 diameter rod inserted in the jet.

It is these numbers that we will use in all comparisons when making jet changes. We will use these numbers also to look at the percent differences in jet changes.

So that you won t need to run around with a calculator, my carburetor Listing in Table 1 shows the metering areas for every carburetor listing at both cruise and at WOT. The number is the metering area in thousands of an inch for a single jet/rod in the carburetor. This number is effectively how rich/lean the carburetor is really jetted, and you can directly compare these numbers to see how the various carburetors were set up by the factory. By dividing one area into another area, you can see the percentage difference in the jetting.

Figure 2 is a table showing you what the metering area is for every possible jet/rod combination. Each grouping of jets starts off with the rod power tip diameter of .026 so you can see the WOT metering area of that jet size. It then jumps to the first usable rod size.

Tech Tip #1
Before you go trying to fix all the errors of the previous carburetor tuner, set your carburetor up to the stock spec for your carburetor part number. A carburetor jetted and set up to its stock specs will usually run pretty good on just about any application, and this gives you a good starting point. From there, you can start doing refinements as outlined in this paper.

The carburetor number on a Q-Jet is usually stamped into the bowl casting on the driver s side of the carburetor in the area above the secondary throttle shaft. The number starts with either 70 or 170 . If the carburetor is a Q-Jet manufactured under license by Carter, it will sometimes have the carburetor number stamped into a foil circle on the driver s side of the bowl just above the primary throttle shaft.

The carburetor listing (Figure 1) is a partial listing of popular Pontiac and Chevy Q-Jets that I have compiled over the years. It is not a complete listing of every carburetor used by Chevy or Pontiac. Most notably, I have very few of the truck carburetors listed, yet there are many truck carburetor s running around on passenger cars.

Tech Tip #2
What has a greater effect on performance: primary or secondary jetting? I constantly see people swapping around secondary rods, trying to get the best performance out of their cars. The secondary rods are very easy to change, and since the secondaries are so BIG, the secondary metering has to be the most important, right?

Wrong.

Most Q-Jets are 750 cfm carburetors. This is more airflow than most small block engines can ever handle. Yet, GM used Q-Jets on everything from Overhead Cam 6-cylinder Pontiacs and Corvair 6-cylinders, to 500 cube Caddys. How?

The secondary air valve on the Q-Jet effectively makes the Q-Jet a variable-cfm carburetor. The spring windup of the air valve combined with the bleed-off of the choke pull off diaphragm allow the secondaries to open only as much as the engine can handle. Thus, if the engine can t handle all of the cfm, the secondaries simply don t open all the way.

The primary side, however, is used throughout the rpm range. It is always in use, and provides the metering for the majority of the power produced by the engine. Let s look at the scenario:

You re at the stoplight. You bring the rpm up slightly against the torque converter 1500 rpm. You re on the primary side of the carburetor only, and this is what is producing all of your torque right now. The light changes, and you put the pedal to the metal. All of your torque at launch is being produced by the primaries only, as the secondaries don t see enough airflow to open. The rpm comes up quickly: 2000, 2500, and now the secondaries might be starting to crack. Almost all of the air is still passing through the primaries, and the secondaries are now starting to compliment it just a tad. 3000, 4000 rpm, and the secondaries might be half-way open. The primaries are still providing most of the airflow and metering. 5000, 5500 and you hit red line just as the secondaries hit about ¾ open. Second gear, your rpm drops, partially closing the secondaries back up, and you re back to sucking the majority of the air through the primaries once again.

So we see, the secondaries provide only a compliment to the primaries. The primaries provide the vast majority of the fuel metering, and primary jetting is absolutely the most critical to proper performance. You cannot compensate for poor primary jetting by re-jetting the secondaries. So we are going to concentrate on jetting the primary side for peak performance, and then we will set up the secondary side to provide a proper compliment to the correct primary jetting.

Tech Tip #3
How can you tell if an off-idle stumble is caused by a lean or a rich condition?
A carburetor running rich, as well as a carburetor running lean, can cause an off-idle stumble or hesitation upon acceleration. To narrow it down, tap the roll pin out of the accelerator pump lever by using a small pin punch or a small finish nail. I actually use a small, broken drill bit that s just the right size. Using a hammer, gently tap the roll pin in towards the choke air horn wall. Don t jam the pin right up against the wall: Leave just a little bit of a gap so you can get a screwdriver blade in between the wall and the pin to pry it back again. With the pin tapped out, remove the accelerator pump lever. I like to do this with the engine running so I won t have any trouble starting the engine without the accelerator pump. Now, rev the engine a little with the throttle. Notice if the engine seems quicker and more responsive, or if the hesitation & stumble is worse. If the engine actually feels more responsive with the accelerator pump disconnected, you have a rich condition. If the hesitation is worse than before, you have a lean condition. If there is no change whatsoever, you have a non-functional accelerator pump.

To verify a suspected lean condition after this test, simply hold your cupped hand lightly over the choke air horn area with the engine running at idle, restricting the air flow. If the idle speed and idle quality momentarily increases, you have a verified lean condition. You need to select a jet/rod combination that will give you a little more Cruise Metering Area. Make these changes in less than 10% increments using the Figures provided in this paper.

Tech Tip #4
How can you tell if your power piston spring is too stiff and not allowing the power piston to seat at idle?
If your engine does not produce enough manifold vacuum at idle and/or cruise (due to a lumpy cam or other engine parameters), it is possible that the power piston is not being pulled all the way down to its seated position due to the power piston spring being too stiff. The result is that the car will run very rich at idle, and the idle mixture screws will have little effect or response. Idle speed may also float, with idle speed starting high and gradually decreasing until the engine stalls due to the engine getting loaded up. There will typically be a puff of black smoke out the tailpipes when you flick the throttle.

To test for this, pop the top off the carburetor, remove the power piston/rod assembly, and remove the power piston spring from its bore. Re-install the rod/piston assembly without the spring and put the carburetor back together. The carburetor will now run in the full-lean condition all the time. If this clears up the idle, improves idle mixture screw response, and eliminates the black smoke when you flick the throttle, you need to install a softer spring. Edelbrock has a complete power piston spring assortment available.

Tech Tip #5
How can you tell how stiff the power piston spring needs to be, and how can you tell one spring from another?

If you have a few springs of various kinds laying around, it is not readily apparent which spring is stiffer than another. You can arrange them and order them from softest to stiffest as follows:

Using your carburetor, or a junk float bowl from another carburetor as a testbed, remove the carburetor air horn (the top of the carburetor) and remove the power piston and its spring. Remove the primary metering rods from the piston. Now, drop a spring into the power piston bore and install the piston. Find a Phillips screwdriver, and place the handle of the screwdriver on top of the power piston with the shank of the screwdriver pointing straight up. Use a screwdriver that is light enough to NOT compress the power piston and its spring, but close. Now, drop flat washers onto the shank of the screwdriver and keep stacking them up until the piston compresses the spring and seats in the bore. Count the number of washers it took to compress the spring and label the spring as a 6-washer spring, for instance. Do the same with the other springs you want to test. You ll end up with a comparative rating of springs, like 4-washer, 6-washer, or 10-washer springs. You now know exactly how to arrange them from softest to stiffest.

But which one should you use? You ll need a junk Q-Jet float bowl for this test, and you ll need to have your engine in running condition.

Using a stripped down, bare Q-Jet float bowl, you ll notice that there is a hole in the bottom of the bowl right underneath the power piston bore. This is the vacuum hole that applies manifold vacuum to the power piston. Hook up a long vacuum hose to a manifold vacuum source on your engine. Now, install a power piston spring from your arranged spring selection into the piston bore and install a power piston on top of the spring. Start your engine, and stick the end of the vacuum hose onto the hole in the bottom of the stripped down float bowl. With the engine at idle, the vacuum applied to the bowl should immediately pull the power piston down against the spring pressure and seat the power piston firmly in its bore. If the piston does not fully seat, you need a softer spring from your arranged spring selection. If you have an automatic, put the transmission in drive. Make sure the power piston stays seated.

If you really want to do some testing, you can string the vacuum hose into the car, and with an assistant, drive the car around and observe under what conditions the power piston starts to unseat: While you drive, have the assistant stick the vacuum hose onto the bottom of the bowl, and observe what the piston does under various engine loads. Make sure you have a spring that s stiff enough to make the piston pop up when your engine is under load, yet soft enough to keep the piston fully seated at idle, at cruise and under light acceleration. This makes for some really fun testing, and the results will pay off in a precisely matched power valve spring for some outstanding throttle response.

Of course, if you buy the power piston spring assortment kit from Edelbrock, the springs will be identified and labeled as to their vacuum rating. Select and use a spring with a rating about 1.5 to 2 lower than the idle vacuum of the engine (in drive).

Tech Tip #6
The idle metering circuit on a Q-Jet is not an independent, stand-alone circuit. The idle mixture screws in the throttle plate receive their fuel through the main metering jets. Thus, a change in the main metering circuit (jets and/or rods) will affect the idle circuit. The idle mixture screws cannot meter more fuel than the main jets/rods will allow. Thus, if your Cruise Metering Area jet/rod combination is too lean, you may find that your idle mixture screws are ineffective. If your idle surges, is rough & unstable, and adjusting the screws seems to make no difference (but you can kill the engine by turning them all the way in), chances are good that your cruise metering area is too lean. You can verify this by running your mixture screws out to the point where additional turns have no effect on idle. Then cover the choke area of the carb with your hand. If idle speed & quality increases as you restrict the air flow, your jet/rod combination is too lean.


Procedure
Here is my recommended sequence and procedure for doing a basic Q-Jet set-up:

1. Set the float level.
You ll be amazed how many people try tuning a Q-Jet without ever checking the float level. An incorrect float level can give you all kinds of symptoms and problems, so get this one set right off the bat.

You have to pull the top of the carburetor off to set the float level. With the top removed, remove the big phenolic spacer that covers the area around the needle/seat. Hold the float hinge clip firmly seated and push down lightly on the float where it contacts the needle. Measure from the top of the float bowl to the top of the float at the rear edge of the float. Float level should be .375 - .400 for a street-driven car. Adjust by removing the float and bending its lever arm. Never raise the float level by forcing the float against the needle/seat to bend it this will damage the needle.

2. Determine main jet size.
If you have a stock engine, always start with the stock jet size and work from there. If you have the typical street modifications like headers, good exhaust system and a free-flowing intake, you can start with a main jet size 2 sizes larger than stock.

Since we want to work on the primary side only, we don t want the secondaries interfering with the jetting process. Chevy Q-Jets have a secondary lockout lever on the passenger side of the carburetor right at the secondary throttle shaft. This lever is actuated by the choke linkage, and prevents the secondaries from opening when the engine is cold. I call this the “primary jet tuning lever. Use a piece of wire or string to engage the lever with the secondaries so that the secondaries cannot be opened.

You now need to find a short flat stretch of road to test drive the car. You need to be able to measure time-to-distance and/or speed-at-distance. I usually find a repeatable stretch of road about 300 feet long. This gets me through 1st gear and into 2nd. Make two or three runs on the car through this stretch and make note of time and speed to distance. Also note the seat-of-your-pants feel of the car s going to feel pretty slow with the secondaries locked out

I recommend making jet changes in less than 10% increments. Go to Figure 2 and determine your WOT metering area for your current jet size. This will be the metering area of the jet with the .026 rod. With this number, go to the Jet % Change Chart and find the closest metering area match in the left vertical Metering Area column (Use the Area column and not the Jet Size column. The Jet Size column can only be used on carburetors that do not employ a metering rod, such as Holley and Weber.). Follow the row across until you get into the green zone and find the closest number to 10%, but not greater than 10%. Now go straight up until you get to the new metering area number. This is your target. Take this number and go back to Figure 2 and find the closest jet size that will produce this metering area with a .026 rod. This is the first jet size you want to try, and this will increase your fuel mixture by the percentage indicated in the chart.

Now, to keep your off-idle mixture unaltered, you also need to check your cruise metering area. Go to Figure 2 and find your old main jet & rod combination. Note the resulting metering area for this combination. Now, go to your new main jet size that you re going to be using and find the rod needed to produce the same cruise metering area you had before. Use this rod with the new jet.

By doing this, you are now changing only 1 parameter at a time: WOT mixture only. Idle, off-idle, and everything else is now unchanged, and you will be able to see the results from the mixture change at WOT only. With the secondaries still locked out, run the car 2- 3 times down the same stretch and record results. If the numbers get better, you re going the right way with the main jet size. If the numbers are worse, you need to make changes to the lean side instead of rich. Repeat this operation until you determine the main jet size that produces the best numbers. On many stock cars, you may be surprised to learn that you end up with the stock jet size. You have now optimized main jets.

3. Determine main metering rod size.

NOTE: There are two different series of primary metering rods. Q-Jets up through 1974 (the 4MV series carburetors) use the early series rods, also known as the single taper rods. 1975 and later Q-Jets (the M4M series carburetors) use the second series rods, also known as the double taper rods. Not only do the rods differ in their taper design, but they are different lengths. You cannot interchange the two different rod series.

When switching main jets around, you were also swapping out metering rods to keep the cruise metering area unchanged. You did this to make sure that your off-idle throttle response remained unchanged so that the throttle response off idle did not affect the tuning results from the main jet re-sizing. Now, with your new main jets, your cruise metering area is exactly the same as it was before, but that s not to say it s right.

There are several indicators of correct cruise metering area. First, check out Tech Tip #5 regarding the idle circuit. This is a good indication of a lean condition. But here s another good indicator of correct cruise metering area:

A Q-Jet, when set up with the correct metering rod for cruise & idle, will produce a slight hesitation upon acceleration if the accelerator pump is disconnected. Using a small pin punch or a finish nail, carefully knock out the roll pin securing the accelerator pump arm to the top of the carb. I do this with the engine running so I don t have any trouble starting the engine without the accelerator pump. With the pump disconnected and with the engine running in neutral, flick the throttle just a little. If the engine actually feels more responsive with the pump disconnected, your cruise metering area is too rich, and you need to install a fatter set of rods. If you get a severe stumble, or if the engine dies, you re on the lean side and need smaller rods. When the rods are correct for the jets in use, you will get a slight hesitation when the pump is disconnected.

Once you have set the rod size up like this, verifying both the idle as shown in Tech Tip #5 and using the disconnected accelerator pump, a road test is in order. If the car is a little flat on light acceleration, or if it has a slight surge at steady cruise, you need to richen up the metering area slightly. If it is smooth and responsive on light acceleration, and feels smooth at cruise, you have the rod size nailed down.

Again, use the charts to keep all changes limited to 10% at a time. This will prevent you from over-shooting. Remember, with the main jet size determined, your rod sizing is affecting idle, off-idle, light acceleration, and cruise. In most cases, when there are problems with stumbles, poor idle, and surging at cruise, the rods are too big and are causing a lean condition. On the other hand, if the rods are too small, causing a rich condition, the throttle will feel lazy or slow when you rev the engine, and you may get a puff of black smoke when you flick the throttle. Correct rods will produce crisp, clean and instant throttle response.

4. Determine secondary rod size.
You are now finally ready to unlock the secondaries. But before you start changing the rods, you want to get the secondary opening rate set up. This is determined by the spring windup.

It is a very common speed trick to loosen the secondary windup spring so that the secondaries will open very quickly. This is the single most common cause of a severe stumble or hesitation upon acceleration or transition into the secondaries.

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carburetor, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening.

Now, you need to adjust the secondary rod hanger height. You ve read all about the different letter numbers for the secondary hangers, and how a Y hanger will make your car faster than an M hanger or whatever. Fact is, you can bend and adjust any hanger to any hanger height you want, so it doesn t make a heck of a lot of difference what hanger you choose to use. Just get it set up right:

With the secondary air valve held wide open and the secondary rods pulled all the way up, measure the distance from the top of the rear wall of the choke horn to the secondary rod hanger hole in the hanger. This distance should be 41/64 Bend the hanger to adjust you have to adjust each of the two sides independently. You now have a performance rod hanger.

With this set, you can now play with secondary metering rods. A common speed trick mistake is to always install thinner (richer) secondary rods. Some engines and carburetors will produce a secondary lag if the rods are too thin. On about half of the engines I work on, I obtain better performance by installing fatter non-performance rods. Again, a quick road test is the only way to set this up, so go back to your 300-foot stretch and make a few runs with rods both richer and leaner. Once you have found the rods producing the smoothest secondary transition and the best numbers, you can start unwinding the secondary air valve spring. Relax the spring tension in 1/8 turn increments until the car stumbles on acceleration, then tighten up 1/8 turn again. You have now determined the quickest secondary opening rate that your engine can handle, and your secondary mixture is set.

Note that secondary metering rods come in three different tapers: long tip, short tip, and medium tip (see Figure 3). Most of the available after-market metering rods have the long tips, and these will produce a full-rich mixture upon the slightest opening of the secondaries. Many street engines will produce better performance by using the short tipped rods. A short tipped rod does not allow a full-rich mixture until the secondaries are opened quite a ways, keeping the mixture a little lean initially. This can produce smoother and crisper performance in many applications. Next time you see a junk Q-Jet laying around, make sure you yank the rods and jets out of it: many old truck carburetor have some really good short-tipped secondary rods in them. Figure 3 lists all the secondary rod letter codes, part numbers, and measurements.


Parts
If you don t have a stash of used Q-Jets in your basement to rob jets and rods out of, you can get parts from Edelbrock. Your local parts store should be able to order them for you. Following is a partial listing of Edelbrock Q-Jet parts and part numbers:

Primary Metering Rods (pairs) for 1974 & earlier:

.035 #1936 .039 #1939 .043 #1944
.037 #1937 .041 #1942 .045 #1946

Primary Metering Rods (pairs) for 1975 & later:

.048 #1941 .052 #1945
.050 #1943 .054 #1947

Secondary Metering Rods (pairs) for all years:

CC #1950 CK #1952 CL #1954
CE #1951 AY #1953

Primary Metering Jets (pairs) for all years:

.068 #1968 .072 #1972 .076 #1976
.069 #1969 .073 #1973 .077 #1977
.070 #1970 .074 #1974
.071 #1971 .075 #1975
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: q-jet info

Postby grumpyvette » March 10th, 2012, 4:20 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Location: florida

Re: q-jet info

Postby grumpyvette » September 20th, 2014, 8:50 am

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Posts: 14105
Joined: September 14th, 2008, 1:40 pm
Location: florida


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