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Q-Jet articles

It makes me crazy when I bookmark a site and go back later and it's gone. So I'm going to mirror some of the better Q-Jet articles here. I will provide links back to the sites where I got them at.


Q-Jet parts

Lot of Q-Jet stuff from corvettefaq

Some stuff from BuickPartsDirectory

The article below is from this site.

Quadrajet Tech

By Randy Frazier

As a service to Dallas Area Pontiac Association members and in the spirit of the Pontiac Southern Nationals, this article has been prepared with the hope that it will help enthusiasts optimize their Quadrajet carbs for performance. In my opinion, Quadrajets are the best four barrel carburetor ever produced, but lack of information on how to tune them has resulted in many enthusiasts replacing them with other types of carbs. While hundreds of articles have been written on Quadrajets, very few actually give specific combinations that perform well on street/strip applications. It should be noted that Rochester designed the Q Jet, and built most, but during times of very high usage, Carter also built Q Jets for Rochester, and to Rochester specs. Carter and Rochester Q Jets are identical in design, operation, and parts usage.

We will assume that the reader already is capable of, or can learn from other sources how to overhaul the Rochester Q-Jet, so we will deal strictly with parts availability and tricks that have been proven to work on street/strip performance applications.

First, you need to understand that the Quadrajet, unlike the Holley carb, is an "air valve" carburetor that senses air flow requirements of the specific engine and meters fuel to those requirements. This is why, although rated at either 750 or 800 CFM, they could be used on a variety of engines ranging from a Pontiac OHC six cylinder to a Super Duty or Ram Air engine. For the time period that they were introduced (1965) the design was very high tech and it is still very satisfying to be able to blow the doors off of one of those computer controlled fuelies with a carburetor that may very well be older than the driver of that 5.0 liter Mustang in the other lane!

Outward appearances of all Quadrajets are very much alike, but from the factory, subtle changes were made in metering and casting orifice sizes to better match the carburetor to the specific job it had to do. Not all Quadrajets are high performance models, but by performing some of the basic "tricks" explained in this article, one can close the performance gap between a truly high performance carburetor such as was OEM on the '69 Ram Air engines as compared to the Quadrajet that left the factory on a '75 model "smogmobile". The tuning systems are quite simple, there are no power valves to rupture, and the few inherent shortcomings of the OEM design are easily overcome. Quadrajets have been made to work successfully on super stock cars running well into the 10's. This is not to say that replacing a Quadrajet with a replacement carburetor is a poor choice, but the point is that your poor performing Q-Jet can probably be made to run well, if you are armed with the basic knowledge to do so, so buy a couple of cans of carburetor cleaner and a good quality overhaul kit, a few basic tools, and some of the parts listed here, and go to work!

One very important point to remember is that ignition system problems diagnose almost the same as carburetor problems. Make sure that your ignition system is in tip-top shape or you will never know if you have carb trouble or not, and I stress that this includes the mechanical and vacuum advance systems of the distributor. Also, leaking casting plugs should be sealed with epoxy and severely warped air horns (caused by years of over-tightening) should be replaced.

The three basic systems of the Q-Jet are (1.) idle circuit (2.) primary circuit which includes the power enrichment system and (3.) the secondary circuit. First, in regards to performance measures, not much is required or desired from the idle circuit except that the engine speed at idle be fairly constant at a high enough RPM to pull a decent manifold vacuum. This will prevent the carb from "loading up" which is caused by a weak vacuum signal to the power piston. If manifold vacuum is too low, simply put the engine "thinks" that you are accelerating or under load so the power system "cuts in" and richens the mixture which causes hesitation, plug fouling, and loss of performance "off idle" (poor 60 ft. times). How can you keep your idle system working properly? A few tips: Vacuum leaks from worn throttle shafts or carb gaskets can cause poor idling. So can "too big" of a cam, or the lack of or a malfunctioning distributor vacuum advance. I recommend always to use a vacuum advance tailored to your engine requirements. One other thing that can cause poor idling on late 60's GM cars is this: Some GM vehicles in '67 and '68 used an open heat riser trough cast into the intake manifold underneath the carburetor. These manifolds required a special gasket to seal this trough, otherwise the exhaust gasses would overheat the fuel bowl causing percolation and very poor idling. Using the improper base gasket would also result in the gasket burning out, causing vacuum leaks with little warning. If your car has one of these intakes, use the proper gasket!

Assuming that the idle system is in decent enough condition to meet the above requirements, lets begin with the actual modifications. We'll begin with the primary circuit, fuel bowl, and accelerator pump circuits.

The worst inherent problem to overcome with the Q-Jet is the small bowl size. In stock form, there is barely enough gas stored in the bowl to take a very strong running Musclecar from one end of the quarter mile to another. Raising the fuel pressure available to the carb is not the answer as Quadrajets can work fine with as little as 3 PSI inlet pressure, but the volume of gas must be there! If you are sure that your fuel pump is capable of keeping the bowl full under several seconds of WOT conditions, and your gas tank has sufficient fuel to keep the pickup tube covered under acceleration, proceed to the following:

First, remove the stock filter that GM used in the inlet fitting (bronze or paper) and replace it with a large in-line filter that presents less of a restriction. Next, remove the stock needle and seat and replace it with GM part number 7035142. This needle and seat has large "windows" and a large .136 size inlet orifice for increased flow capacity. These were original equipment on the Ram Air GTOs and Firebirds in the late 60's and were also used on the Buick Stage II modifications as well as other GM Musclecars. They are still available from GM, but must be ordered in lots of 10.

Next, remove the plastic "baffle" that is located in the area around the power piston. Do not leave the baffle out, but we are going to modify it in order to increase fuel bowl capacity. I use a coping saw to cut about one quarter of an inch from the bottom of the baffle, then use a sanding block to smooth the surface. Some of these baffles are plastic and others are phenolic, so be careful; and do not break them as they are brittle. The purpose of this modification is to remove some material that takes up space that would be better occupied by fuel.

While we are on the subject of fuel bowl volume, it's worth mentioning that very early production Quadrajets used larger floats than the later models. The reason was that the pivot point for the float was located further inboard and the float had to be large enough to gain enough leverage to hold the needle on the seat. This arrangement was modified in the late '60's by moving the pivot point forward. This allowed the use of a smaller float which consumed less space in the fuel bowl. Make sure that the float used is not larger than it has to be. Also, these early Q-Jets used a plunger type needle and seat that was so unsuccessful that they were replaced by GM in early '68.

Even the overhaul kits had a modified needle and seat to replace the factory unit. The only way you will find one of these now is if you find an original carb that has never been overhauled. I have only seen two of these needle and seat setups. One was in a '67 Chevy Impala and the other in a very early production '68 GTO.

Now to turn our attention to the accelerator pump circuit. Remove the plunger and then remove the cup and spring. You can shorten the tip of the rod where the pump actuator lever rides by about one eighth of an inch. This will allow the plunger to ride slightly higher in the well and will help prevent the available "pump shot" from being used up before leaving the line, when the throttle is opened against the engine torque converter or clutch on the

starting line. Reinstall a new spring and cup. With the air horn still removed, and the air horn gasket in place on the bowl, locate the discharge passages for the pump which are machined into the bowl casting. If you compare the size of the actual orifices to the size of the matching holes in the gasket, you will notice that the gasket holes are smaller than the actual orifice size. Enlarge these holes in the gasket so as to not restrict the accelerator pump shot.

The next step in your modifications should be considered one of the most important. The power enrichment circuit can make or break off-idle acceleration and has an effect on emissions as well.

To some enthusiasts, changing primary jet size is quite possibly the only modification made. However, changing jet size without taking into consideration the size of the corresponding primary metering rods or power piston spring tension can be totally useless and possibly can hinder performance.

Under actual operating conditions the power piston meters additional fuel by raising and lowering metering rods in and out of the primary jets. The motive force for this movement is engine vacuum and the resisting force is the tension of the spring located underneath the piston. If vacuum is too low for the spring tension then the power piston will be pushed upwards, which raises the rods out of the jets and richens the fuel mixture. This is normally what happens at WOT (wide open throttle) acceleration. However, if the spring tension is mismatched to the engine vacuum (camshaft) or if the metering rods are mismatched to the jet sizes, then performance will suffer. GM used many different jet/metering rod combinations. However, some of the metering rods are no longer available from GM, but at the time of this writing, availability of jets is not a problem. At the risk of oversimplifying I'd say that 95 percent of all Pontiac combinations will run well on primary jet sizes ranging from .070-.074, providing that the corresponding metering rod sizes are about 30 thousandths smaller, give or take a thousandth or two. Example: A primary jet size of .074 should be matched to a rod size of 44 (.044). If you go up or down over 2 or 3 thousandths, then you need to change rod size too. For those that must pass emissions tests for inspections, you should really pay attention to this detail. GM made it easy to tell the sizes of the jets and rods as the size is stamped onto the rods and jets themselves. Also, the last two digits of the GM part number corresponds to the size. Example: GM jets part # 7031974 corresponds to a size of .074.

To summarize this section on primary rods and jets, don't concern yourself too much with trying to increase horsepower by using a larger size primary jet. That can be accomplished with modifications to the secondary circuit. Your goal here should be that off-idle acceleration should be crisp and that the engine should not lean out under high manifold vacuum conditions, such as cruise speeds. This can be detected by a slightly jerky uneven feel at steady cruising speeds. This condition is usually worse on '75 and later emissions model Q-Jets and those equipped with EGR valves (exhaust gas recirculation). The '75 and later Q-Jets also used different power pistons and rods which will not interchange with '74 and earlier models, but the jets and other parts will. Also, on '75 and later model Q-Jets the factory used a couple of versions of APT devices (adjustable part throttle). The models can be identified by the extra bulge cast into the front of the air horn. The purpose of the APT was to provide a leaner than normal mixture under high vacuum conditions. The factory literature that I've seen advised the GM mechanics to not change the settings, and really contained no other service information than to replace the device if it was malfunctioning. My experience with '75 and later Q-Jets is that they are not as good for performance applications as the earlier models, and may not run as smoothly because the APT may be out of adjustment. For performance applications, I always suggest replacing them with '74 and earlier models, but if this is not practical, you may can change to a slightly larger jet size and smooth out the lean condition somewhat.

While on the subject of replacing carburetors, beware of buying rebuilt models at your local parts dealer. Some of these rebuilders may render the carb useless for performance modifications by their techniques. Usually, the jets and rods that they use are generic and not GM quality parts. I have also seen rebuilt Q-Jets with the needle and seat fixture drilled all the way out and the seat assembly replaced with a press in assembly sealed with an O-ring. If a procedure like this has been done on your carb., you can not convert to the high flow needle and seat as they are meant to be threaded in.

Concerning power piston springs - a point to remember is this:

Camshafts with low vacuum characteristics need a matching spring with weak tension. This is because if a strong tension spring is used, the spring will overcome the weak vacuum signal and cause the power system to cut in prematurely. GM part # 7029922 is a rather weak spring that was OEM on early '70's 455 engines including the 455HO. The power system cut-in is delayed until manifold vacuum reaches 3 in. Hg. No doubt this was done for emissions purposes, but the good point is that this spring will work well with a rough idling camshaft. I have used this spring on cams with 230 degrees of duration and it works well. Another spring that may be considered is GM part # 7037305. The power system cuts in early on this spring, at 6 in. hg, and it is a good choice for a very mild camshaft, such as some of the milder factory cams. Both springs are still available from GM, but only come in packs of ten.

Now we move on to modifying the secondary side of the Q-Jet. This is where the "real" horsepower is made but it is no greater in importance than the other systems we have discussed, because all systems must work together.

First, make sure that the secondarys open all the way. On some early model Firebirds the secondarys were only designed to open to about 70% at WOT. GM had horsepower constraints on it's cars in the 60's (except for a small group of backdoor models) which generally limited engines to 10 lbs. Per horsepower. The 400 cubic inch engines being installed in the Firebirds were the same engine used in the GTO, but the cars were a few hundred pounds lighter, so the carb restriction resulted in reduced horsepower levels. If you have one of these neutered Q-Jets, you will be better off changing base plates or swapping carbs.

Make sure that the fuel delivery tubes which are pressed into the lower part of the air horn have not become loose and fallen out. It is not uncommon to find one of these laying in the fuel bowl on disassembly. You can tap it back into place with a little Loctite used sparingly, just don't plug the tube.

Remove the secondary lockout tang if your carb has one. They are held in with a small roll pin. This device is located on top of the air horn. The purpose is to prevent the secondarys from opening when the choke is engaged, but they are also guaranteed to prevent the secondarys from opening, even when you want them to! One extreme case of misfortune that I am aware of was when a roll pin worked its way out on a friends '69 GTO, and the lock out tang managed to get swallowed by the secondarys on the carb!

Air valve opening rate is controlled by two devices. One is the air valve diaphragm which may be plastic or metal. Either way, enlarging the vacuum port orifice size will allow the secondary air valve to open sooner on acceleration. The other device is the tension spring that is attached to the air valve shaft. The tension can be adjusted with a small Allen head wrench and screwdriver. Adjust the valve by loosening the Allen head screw and then tightening or loosening the screw. The object is to find a compromise between the valve opening too fast and causing a bog or opening too slowly and costing some E.T. The only way to tell if you have it right is to experiment, but if in doubt, it is better to have it a little tight than too loose. Some "performance" versions of Quadrajets were built with "transition slots" cut into the air valves which made for a smooth transition from primary to secondary operation. If you can find some of these, use them!

Another trick worth trying is to rough up the bottom of the secondary air valve plates with a rough file. This can aid in fuel atomization starting early as the fuel discharged from the secondary fuel tubes strikes the bottom of the plates first.

We are nearing the end of this lengthy article and still have not mentioned secondary metering rod choices, but we will do so now.

First, inspect the hanger that the secondary metering rods attach to. If it is an original hanger, chances are the little holes that the rods are suspended from are worn egg shaped. This will allow the rods to hang lower into the secondary jet orifices than they should, which in turn can throw off the air/fuel ratio calibration. GM used many different hangers down through the years. The idea was to vary the A/F ratio by changing the hanger height. The letter designation on top of the hanger represents how high or low the

secondary rods were dropped into the jet. Each hanger designation from "B" to "V" lowered the rod into the jet by .005 inch. Unfortunately, all of the hangers are not available from GM now. Replace your worn hanger with GM # 7034522, which is a "K" hanger. This hanger may not be identical to the one you have now, but it will work fine and it is better than a worn out part. Refer to the instructions included in your carb kit, and set the hanger height

by bending the tang on the hanger. The measurement should be made from the top of the choke horn to the top of the metering rods with the secondary air valves held wide open. Set the height at 41/64th.

The final portion of this article deals with secondary metering rod selection. GM produced many different rods for different applications. The rods are identified by a letter designation stamped onto each rod, but this is not to be confused with the GM part number, which is not on the rod. Typically, most OEM rods produced after 1970 were in the 50 thousandths size range. These are much too lean for a healthy 455, but possibly could work well

on a smaller 350. Experimentation is the key here. Most Pontiac drag racers are running 400 or 455 combinations, so I'll list a few rods that work well for me. However keep in mind that atmospheric conditions may be cause to deviate from what I have listed. Here in Texas, most of the drag strips that I run on are from near 300 ft above sea level and lower. Ambient temperatures range from the 60's in early spring and fall to near 100 in the summer. With these conditions in mind, here are my choices for 400/455 combinations:

7042356 - letter code "CC" (.030 tip) Cold weather.

7033549 - letter code "AX" (.040 tip) Moderate weather

7046010 - letter code "DA" (.044 tip) Hot weather

Generally speaking though, if you stick to tip sizes in the .040 range, you won't be far off.

In conclusion, what kind of performance potential should you expect for a Q-Jet set up this way? I suppose it depends on your overall combination. My 455 GTO with 3.42 gears and shifting at 5200 RPM has had no problems turning 12.40's at nearly 109 MPH and this is a car that weighs nearly 3800 pounds, with a mild converter. This is in a car that uses stock fuel lines and pickup tube in the fuel tank, and using only a mechanical type (Carter) pump. The best part of these modifications is that the cost of the parts generally is below $100.00! Good luck!

Randy Frazier

Dallas Area Pontiac Assn.

Member # 187

Q Jet Addendum By Jim Hand

The above article is an accurate and practical review of the Q Jet, and is very much a duplicate of what we have done with our Q jets. In particular, Randy’s suggestions on secondary metering rods for a 455 are close to what I run in my wagon. Pontiac used BE rods (.041) in some applications, and they also work well. Concerning the secondary rod hangers, there are many versions that position the rods higher or lower in the secondary jets. For drag racing, they are not terribly important because we normally go from idle or low speed to full open throttle. Thus the hangers move the rods from the lean condition to full rich almost immediately. For normal driving where the secondarys may need to be partly open (such as a GM engined motor home or road racing), the hangers become very important because they determine at what point the fuel mixture changes to a richer condition via the tapered section of the metering rods. Randy advised to be sure that the secondary throttle plates open fully. Also check that the air valve opens adequately. When looking straight down from the top, the front edges of the valves should be at approximately the center of the air valve shaft when fully open. Two things that control the maximum opening are the vacuum pull-off rod, and the mechanical stop on the air valve haft at the opposite end of the pull rod slot. Bend/adjust the pull rod, and/or file the mechanical stop. My air flow bench showed about a 3% increase in air flow through the carb after this mod. As Q Jets are air-on-demand carbs, and are effectively variable size, this may not translate into an actual performance increase, but it does provide more potential air flow if our engines actually need it.

He also suggested using the air valves with "transition" slots. Most Pontiac Q-Jets have small orifices (ports) located directly in front of and at the center of the air valves, called the accelerating ports. These ports function as accelerator pumps for the secondarys. When the secondary throttle plates begin to open, a vacuum is reflected up into the carb, and this causes a small amount of air to begin to flow around and through the air valves before they begin to open. The transition slots are intended to concentrate that initial air flow past the accelerating ports. As you will all remember from your physics classes, air flow across an opening will lower the pressure at the opening. The fuel inside the accelerator wells, tubes, and orifices immediately begin to travel towards the low pressure area and is sprayed into the air stream at the transition slot. This initial shot of fuel is intended to prevent bog until the secondary metering system begins to meter fuel through the secondary discharge nozzles. This is a great system, and normally works well on stock carbs.

One of the changes Randy mentions is to open the air valve quicker, because this provides quicker secondary throttle response. At some point in the increased air valve opening rate, there may not be enough fuel from the accelerating ports (or it may not begin quickly enough) , and the dreaded bog reappears. To cure this problem, we have made the transition slots smaller. We obtained non slotted Chevy air valve plates, and cut much smaller openings in them (about 3/8" wide and 1/4" deep centered on the accelerating ports). Although the actual size is not terribly

important, both plates should be identical. The smaller openings concentrate airflow and increases the velocity at the accelerating ports, and results in much quicker and stronger fuel flow from the ports. Again, while this change may not result in measurable acceleration improvements, it does sharpen the throttle response and allows the secondarys to begin to add power almost simultaneously with the primaries. Your reaction times, and 60' times will definitely be more consistent when there is sharp and instantaneous throttle response! Before trying to remove the air valve shaft/plates retaining screws, file or grind the ends of the screws where they have been deformed. Using a large blade screwdriver, carefully remove them using a technique of twisting CCW, CW, and again CCW, while using WD-40 for lubrication. These screws may break, but with care, they can be removed intact. When all check-out is finished, a medium strength of thread lock (in case you desire to remove the screws later) should be used to secure the screws in the new plates. Concurrent with this mod., it may be necessary to drill several small holes (about .030") through the secondary well tubes. These holes will allow fuel to fill the tubes faster, and they should be located about 3/8" and 5/8" up from the lower tips of the tubes.

The two fill holes into the wells may also need to be opened up by 1 or 2 thousands. Be very conservative here because the fuel from the accelerating ports continues to feed the secondarys whenever they are open. Perform the last two mods only if a bog still exists after modifying the air valves, or if a bog occurs during a quick throttle opening immediately after doing a long burnout. The sole purpose of the accelerating ports, well tubes, and wells is to furnish the shot of fuel at initial secondary opening, and if it doesn't bog, don't fix it!

Whenever engine modifications are added that tend to adversely affect idle quality, or a later Q-Jet with more emissions controls is used, it is sometimes impossible to obtain satisfactory idle. The two Q-Jet idle "mixture" screws do not actually alter the idle fuel mixture. Instead, they allow more or less of a pre-determined mixture to be directed through the idle discharge ports. The taper angle of the pointed portion of the screws have no effect on the adjustment, other than controlling how sharply they peak (if at all). In many cases, the mixture screws do not provide adequate fuel to obtain a smooth idle. In the pre-emission days, the Pontiac shop manuals specified to adjust the idle screws for "the fastest, purest" idle. In those earlier times, the idle screws had a range that would allow the engine to be tuned from a too lean condition through the ideal mixture, and on to a too rich mixture. The modern carbs are typically calibrated very lean at idle, and almost any change to the engine will adversely affect the idle. Idle mixture is actually determined in the Q-Jets by the "idle tube" orifice size. The idle tubes are located in the main body at the front inner edge of the primary throats. Most of the stock orifice openings range from .030" to .033". We have found that a .037 opening will provide idle adjustment range for almost every application including pure stock vehicles. It is possible to remove these tubes, but it is difficult and requires some special tools. As the actual orifice restriction is located at the lower end of the tubes, and is 1 5/8" from the top, a special 2" length drill is needed. Using a finger operated pin vise, it is possible to drill the tubes without removing them. Coat the end of the drill bit with light grease and very carefully drill through the orifice. The orifice restriction is only about .2" in length, and the passageway holding the orifice tube makes a sharp turn just below the tube. Therefore, proceed slowly and carefully to avoid bending and/or breaking the bit. The light grease will hold most of the brass hips, and the remaining brass can be flushed by spraying WD-40 (what would we do without it?) through all idle passageways in the carb body. Remember that the idle circuit functions at closed, or nearly closed throttle position. To correctly adjust the idle mixture screws, you must concurrently enrich (back out) the screws while reducing the throttle opening (idle speed set-screw turned CCW) . Otherwise, the engine will continue to run on the transition circuits and the idle circuits can not be properly adjusted. The longer drill bits were special ordered and are made by Precision Twist Drill Co. as RN 52063.

I have seen a lot of B.S. about the fuel pressure needed for Q-Jets! As the actual pressure dissipates after the fuel passes through the needle and seat, pressure has no direct effect on the carb operation. The only requirement is that the fuel bowl maintains a high enough level of fuel to correctly meter the fuel at all engine speeds. The carb will function correctly with only 2.5# of pressure providing the fuel system can maintain adequate fuel level. A stock pump generally provides as much as 6 or 7# of pressure at idle, and as PPM increases, the pressure gradually drops down to as low as 1 or 2 #. That relatively low pressure can still keep the bowl adequately full in most cases. Why not run more pressure? Additional pressure blows the float higher and richens the mixture. Excess pressure almost always causes or aggravates a hot start or flooding problem, because it tends to force fuel past the needle and seat, causing the carb to overflow fuel onto the manifold after the engine is shut off. Excess pressure can also cause fuel spewing within the carb and out of the vents.

The fuel system in my wagon is a somewhat overkill setup! The gas tank sits vertically, and I have installed a 1/2" outlet at the bottom rear. The Mallory "140" pump is located at the base of the tank such that gravity feeds the pump at all times. A 1/2" line is routed to the front to the Holley regulator, and a 3/8" line from the regulator connects the 3/8" in-line filter and the carb. This system can provide much more fuel than the engine can use, and the engine will run through the 1/4 mile perfectly with only 2 1/2# pressure. Some time ago, I was testing a non 0-Jet carb and had set the pressure at 6# for that carb. At the drag strip several days later, the wagon was off .5 second and 3 MPH. After 3 passes, I remembered the high fuel pressure, and after lowering it to 3#, the wagon picked up the lost performance. You may not see this kind of drastic change because each fuel delivery system is different. However, I strongly recommend you find out by strip testing what the lowest pressure will satisfy your vehicle, and then set the pressure about 1/2 # higher for insurance. You may be pleasantly surprised, not only with your Q-Jet but with other brands of carbs as well. Recheck for proper metering (rods and jets) after this change. For those of you willing to do a little grinding/drilling/filing/tuning on/with your Q-Jet, the ideas described above might add a bit more drivability and/or performance. KEEP ON RACING!

Jim Hand

Performance Tune Your Q-jet Secondaries

Written by: Damon Nickles
THE SECONDARIES!! Best place to start in search of more performance quickly!

These mods focus on what happens above about 2/3 throttle- that's when the secondaries do their work. The secondaries on a QJet are very very easy to work on:

Secondary air door: Push it fully open with your finger. How far does it open? If its about 80+ degrees, you're OK. If it only opens about 55 degrees, you are losing performance cause the thing isn't delivering its full airflow potential (750-800CFM is what the carb can REALLY do if properly prepped). To get your air valve fully open go to a junkyard and find an older QJet that DOES allow at least 80 degrees of air valve opening. Take the secondary air valve door shaft out of the "donor" carb and install it in yours. All you have to do is take the air valve doors off (they unscrew, 2 screws apiece) and then slide the shaft out the side. Make note of how the little tension/closing spring hooks around the secondary throttle shaft underneath and to the side that you slide the shaft out of. Make sure you hook it back up the same way when you put it onto your carb. Once your carb opens about 80 degrees, you can gradually file down the little stop on the side until you get to about 85-90, where it reaches max airflow. Then just set the tension spring at about 1/2 to 3/4 turn from the zero tension point. Its all the carb you will ever need for street performance. Difficulty level: 2 on a scale of 1 to10.

Now that you have full airflow from the secondaries, you need to MATCH IT WITH THE RIGHT FUEL FLOW! This is the job of the secondary metering rods and hanger. They are located above the secondary air valve doors between the two secondary bores. A single screw holds the hanger on. Unscrew it and lift the hanger and rods out to change them.

(Here's the real scoop on secondary rods: Very few are still available that are any good for performance). A few sets that you might try to find are: DR, CP, or BA. Don't ask why or how, just GET ONE OF THESE SETS and put them in. Here's a co. that sells QJet parts like these- The Carburetor Shop (909)481-5816. That's where I go for all my stuff. ($25/pair). FYI- These Secondary Rods make a bigger diff in performance than all the other mods combined. Stock rods are usually WAY WAY too lean on initial air valve opening and you get the infamous "Quadra-bog." These will fix that. Get 2 hangers while you are at it: B and G. B is full rich, G is just slightly leaner- one of them will be about perfect. ($5/hanger.) UPDATE: They're out of business unfortunately. Some of these rods are still available through GM parts counters but you'll need the part numbers: DR: 17053659, EE: 17081879, CP: 7045842, BA: 7034337, ED:17081878, DH: 7048992. They come individually so you'll need to order 2. I dunno which rods are still valid part numbers and it changes every few months so..... Any ways, any of the above rod sets will do just fine- they're all identical in every way within .0003" (3 TEN-thousanths!) .

Lastly: The secondary air valve "damper." Its the little vacuum can on the side at the front of the carb that also serves to slightly open the choke when the engine first starts. It takes too long to fully open for performance. Most stock ones take about 2-3 seconds (push it in and release while timing with a stopwatch). A hi-po motor should take just under 1 second. Some of them you can modify by drilling out the TINY TINY little hole that acts as the "restriction" in the inlet where the vacuum line plugs on. We're talking a few thousandths of an inch can make a diff. Be real careful and get the right set of (jewelers) drills if you want to try to modify it. OR you could buy one already ready to go from the Carburetor Shop.

That's basically it. Do these mods, with the right rods, and you will definitely be smiling. It'll hit like its got FI and pull much faster out of the hole and a little better on the top end. I would say at least a 1/4 second reduction in ET from these mods alone! That's why I use them- dirt cheap HP if you're willing to spend some time instead of cubic yards of dollars.

Performance Tune Ypur Q-jet Secondaries Part 2

Written by: Damon Nickles
Secondary metering rod measurement and performance is a tough one. You can look at the specs all day long and you won't know what's gonna work or not unless you try a few and start to get a feel for what works and what doesn't. Basically, secondary rods are tapered. They start out at a large diameter and work down to a small diameter at the tip. The large part of the rod's taper is what meters fuel with the air valve closed and as the air valve opens you work your way lower and lower on the taper until you get to the metering tip- at which point the A/V is all the way open and you have max air and fuel flow. Once you're on the tip, no further enrichment is possible with that rod. Rods with "Short" tips reach their maximum enrichment (power tip) at approx. 90+ degrees of A/V opening, medium rods reach maximum enrichment at approx. 80+ degrees and long tips reach maximum enrichment at approx. 70+ degrees of A/V opening. Medium tips are usually a good way to go and are the most "tunable" with a selection of hangers. Speaking of which.....

The hanger does nothing more than determine how far down the taper you START on initial air valve opening (by how high or low the little holes in the hanger are where the rods attach). So the hanger can richen or lean the entire calibration by starting the rod either higher on the taper (leaner) or lower on the taper (richer). The only part of the calibration it DOESN'T affect is once the A/V is fully open and the rod is down on the power tip (maximum enrichment). So the hanger really affects the a/f ratio AS THE A/V is OPENING, but not once it is FULLY open.
Little confusing, no?

The long and short of it is this- almost ALL Q-jet rods are way too lean in the initial 30 degrees of A/V opening (DR rods being a notable exception). Also, the hangers don't make a very large difference in calibration- definitely not enough to make up for a bad set of stock rods. Play around with hangers from "B" (full rich) to "J" (middle of the road) but don't even bother with the leaner ones- they're useless for performance.

The vacuum can is the vacuum, um....., well.... CAN on the side of the carb, passenger side near the front. It uses a little pushrod to connect it to the A/V door shaft arm near the rear of the carb. This vacuum can uses a TINY LITTLE vacuum restriction at its inlet so that the vacuum can releases very slowly. This in turn slowly allows the secondary air door to open. Slowly because if you let it "snap" open the thing will bog badly when you open the throttle quickly. Stock cans usually take 2 seconds or more to open. For performance applications you want one that takes about 3/4 of a second.

Getting a faster opening can is a matter of either modifying your own (TINY TINY little set of drills can do it IF you can even get to the restriction point on your can) or finding one that opens quickly and will bolt onto your particular carb. No easy answers here. You can comb junkyards and look at old Q-jets or you can try putting a vacuum can from an Edelbrock newly manufactured Q-jet on. The vacuum can for an Edelbrock #1910 performance Q-jet should fit your late model Q-jet.
SHOULD! No guarantees.

My Q-jet Tips

Written by: Mike Ervin

To start this off, I just wanted to tell a little bit about the stock Q-jet that came on my truck (1985 C-10). 1985/86 Q-jets in U.S. trucks have dual capacity accelerator pumps. When the engine is cold, more fuel is needed for transition from idle to part-throttle. When the engine is warm, less fuel is required. Dual capacity pump solenoid (plug on top right front of carb) is activated by coolant temperature sensor (sensor in thermostat housing). At 170 degrees pump solenoid energizes, dual capacity pump valve opens and pump capacity reduces by about one-half.
I had performed some of the mods in this article on my stock carb. After it started leaking around the air horn where it had been over-tightened at the carb mounting bolts, I decided to replace it with an Edelbrock Q-jet. The stock Q-jet is an emissions carb as most Q-jets in the early to mid 80's are. These carbs aren't computer controlled as many people think. The only thing electrical about them is the dual capacity pump. The carb has to be plugged to the sensor in the thermostat housing in order to work right. If it isn't, the pump solenoid will stay closed all the time and waste fuel.

I used the articles by Damon on Peformance Tune Your Secondaries, Q-jet Secondaries Part 2, and Q-jet Idle Problems, to help with the tuning of my Edelbrock #1904 Q-jet. The first thing I did was to check the choke pull off/AV damper and see how fast it opened. Well as it turned out it opened real close to one second. Great. The air valves already opened fully also. Next I pulled the sec. metering rods and hanger to see what I had. Edelbrock says on their site the 1904 comes with DR rods. I wanted to make sure. It turns out it was a M hanger, along with the DR rods. I pulled the carb off of the engine to make sure I set the AV spring tension right. Damon recommends to use a B to G hanger. I happened to have an E hanger laying around so I used it and set the tension to zero plus 1/8 of a turn just to be on the safe side. This E hanger was old and a little worn but worked great. I have since replaced it with a new Edelbrock G hanger. No further adjustments had to be made with the change. I found out any hanger from B to G will work just fine.

I have read where people say on the primary side, the primary rods need to be around .030 smaller than the main jets, but this recommendation is for pre-75 Q-jets, without the APT feature. The Edelbrock 1904 Q-jet came with .050 rods and .073 jets. As you see this is a difference of only .023. I was having problems with the exhaust being so hot it was killing my catalytic converter and muffler. After a lot of tuning and guess work I came up with a good combination of a cooler exhaust and good emissions. I kept the 73 jets and installed a set of .048 primary rods and kept the factory setting on the APT at 2 turns out. This stopped the lean condition I was having in the cruise range causing the hot exhaust. I tried a setting of the same jets and primary rods, with the APT set at 2-1/2 turns out. This seemed to work fairly well in cool weather, but as soon as it started warming up, I started having problems. The hotter the weather got, the worst my stumbles got. It would load up while sitting at a light. I went back to the factory setting of 2, and all is fine in truck land once again. By the way, 1975 and later Q-jets used different primary rods than 74' and earlier Q-jets. The 75' and later Q-jets use part numbers that start with 170 (74' and earlier ones start with 70). The part numbers for 75' and later primary rods start with 170513xx, as in the .048 rods I used would be 17051348. The last two numbers are the size of the rods.

The APT (Adjustable Part Throttle) that the Edelbrock Q-jets have is set at 2 turns out from the factory. What the APT does is control how far the primary rods can be pulled down in jets. The power piston has a pin made on it that hits the APT screw and stops it from going any farther down into the jets. The farther out you turn the APT screw, the farther out of the jets the rods will stay and the richer it will run. It is sort of like installing bigger jets because primary rods get smaller the farther up they are held. What you are trying to do here is get a part throttle that is crisp and responsive, without any surging or hesitation. This is not for performance, that is accomplished with the secondary side. You don't want to go too far with this adjustment, because it will just waste gas. Also, if you must pass emissions testing, be real careful. This will increase your emission levels.

There is a way to adjust the APT without having to remove the top of the carb every time. If you look at the air horn just in front of the choke tower where the air cleaner gasket sits, you will see an aluminum plug. To get the plug out you will need to remove the air horn. Once the air horn is off you can knock the plug out from the inside. In order to plug the hole and be able to get to the APT once the air horn is back on the carb, I tapped the hole for a 3/8" x 24 thread allen head plug. This will sit flush with the air horn and not interfere with the air cleaner. Don't tap all the way to the bottom of the hole, you won't the plug to sit flush, so check the depth as you are tapping the hole. Also, you will need to make a tool to fit the APT screw. It needs to be sort of rectangular in shape. I used a piece of 5/16" steel gas line. Just use a pair of pliers to squeeze it into an oval shape, then use a screwdriver to finish shaping it into a rectangle. Test fit until it fits without too much loose motion. You may have to grind down the outside of the piece to fit in the space around the adjustment screw.

On another note, I had to drill out the Idle Discharge Ports (the holes in the baseplate for the idle mixture screws) so I could have an adjustment with the screws. In photo #3 below, the black arrow points to the actual discharge port, inside the throttle bore. This is the hole the pointed end of the mixture screw goes in. This will be what you will drill out bigger.

Chevy Truck Magazine
This is an insert from an article on Q-jet Tuning Tips that appeared in the January 2000 issue.
The holes in the baseplate where the idle mixture screws are located are called the idle discharge ports. These ports, together with the mixture screws, control how much fuel enters the engine at idle and won't need alteration with a stock engine. However, once the engine is modified with a bigger cam, these ports often need to be enlarged to avoid a lean miss at idle. Stock ports are usually 0.050-0.081 inch, and Murphy (Sean Murphy from Jet Performance Fuel Systems) says they can be enlarged up to 0.093 inch, according to engine needs (anything beyond .0093 inch, and you start losing idle-adjustment capability). Although application needs vary, Murphy recommends 0.081-0.086 inch for cams with 204 to 210 degrees of duration at 0.050, 0.086-0.089 for cams between 210 and 216 degrees of duration, and a maximum of 0.093 for cams above 216 degrees of duration. Big-block engines have different recommended specs.

I read the above insert in the article. My CompCams XE268H cam has 224 degrees of intake duration, so I said why not. I checked the holes and found out they were .088 inch. So I gathered up the nerve and drilled them out to 0.093 inch. Worked like a charm. Now I have adjustment with the screws. Before drilling them bigger, I could turn the screws all the way in and not affect the idle. I had to turn them out about 4 to 5 turns to achieve a decent idle mixture. I highly recommend you giving this a try if you have no adjustment with the idle mixture screws. Just for the heck of it, I checked the holes in the old stock Q-jet that I had removed and found out these were, get ready for this, 0.055 inch. That's right, no wonder we can't get a decent idle after we change cams.

Setting The Air Valve Spring Tension
In the photos below are the screws you need to use to adjust the AV spring tension. In photo #1, green arrow, is the actual adjusting screw. Also in photo #1, pink arrow, you can see the rod and lockscrew at the bottom. Photo #2, red arrow, shows the allen head lockscrew. Photo #2, yellow arrow, you see the hook of the spring wrapped around the rod. The rod is attached to the air valve shaft (green) and is how it puts tension on the flaps.

To set the tension properly you need to make sure of zero tension, by this I mean you need to make sure when the air valves are held closed, the hook of the spring is just making contact with the rod that is sticking out of the AV shaft. You have to loosen the allen head lockscrew while holding the adjustment screw, because the adjustment screw can and will come out, believe me. The adjustment screw does not screw into anything, it just tightens the spring, so it will slip out if you're not careful. The best way of doing this is with the carb removed from the engine. This way you can see the rod and spring and will know exactly when the spring makes contact. Anyway, according to how much spring tension you are wanting, once it just makes contact (zero tension), turn the adjusting screw the amount you need and while holding the adjusting screw, tighten the allen head lockscrew. Never tighten the adjusting screw more than about 1-1/8 to 1-1/4 turn or you risk over-tightening and breaking the spring. A real good idea is to make a reference mark on the carb when you find zero tension, inline with the screw slot, so you will know where it is if you need to do this again.

If you don't' want to remove the carb from the engine, you can sometimes get close to zero tension by this method. Loosen the allen head lockscrew while holding the adjusting screw. After you have it loose, turn the adjusting screw counter-clockwise until the A/V flaps fall open. Do this slowly. After the flaps are loose, slowly tighten the adjusting screw until the flaps just close. This is as close to zero tension you will be able to get doing it this way. Now tighten the amount you need to go and while holding the adjusting screw, tighten the lockscrew. That's all there is to it. Our carbs factory setting is 7/8" of a turn past zero tension, if you need a reference or need to set it back to factory specs. (Why would you want it at factory specs?)

My Comments On This Mod

I have run Quadrajets on my cars and trucks for most of the time I have been driving. I have never had one perform like this one does. As Damon says , "It will hit like it has fuel injection". I highly recommend trying this mod to your truck. I guarantee you will love it as much as I do. It is the best thing I have done to my carb and the cheapest. My total cost of the above mod, $2.00 for the Edelbrock G hanger. Well, let me change that, I did have to buy the carb in the first place, but that don't count.

BTW! The Q-jets that came on the L-69 engines (305 HO's that came in early 80's Camaros and Firebirds, along with Monte Carlo SS's) came stock with M hangers and DR rods, most of them anyway. The LG4 engines (basic 305's) had P hangers and DP rods. If you try to find these rods in a junkyard look for these cars for the best chance of finding them.

The part numbers for DR rods is GM17053659. The GM dealer is the only place (besides a junkyard) that you are likely to find these at. They are not cheap, $8.40 each is the price I got from the dealer in my area. On the hangers, the best place I have found is to buy the Edelbrock ones. You can get these at Jeg's and Summit and other places on the net. The part numbers for the Edelbrock hangers are as follows: B hanger #1960, G hanger #1961, K hanger #1962. They have more but these are not worth mentioning.

If anyone needs more info on rods and hangers, as to what all this means, or what size are the metering rods they have, I have all this info if you need it. A good book that everyone who has a Q-jet should have is Doug Roe's Rochester Carburetors. This is The Bible for Rochester carbs, hands down. I can't stress this enough, if you want more performance out of your Q-jet or just want a great how to rebuild reference, buy this book. It is an invaluable piece of information that you shouldn't pass up.

I know that a lot of this will make no sense to you because it didn't to me at one time. I had no idea that you could make a Q-jet perform this much better than stock. As you can tell, I love Q-jet's, it has got to be one of the best, if not the best carb ever built for the street. When properly tuned it will give you many years of great performance and economy. Now start adjusting.

Q-jet Idle Problems

Writen by: Damon Nickles
Quadrajet Idle Quality: Idle quality with a QJet can be tricky since they were ALL emissions carbs (except the Edelbrock's). They can have lousy idle quality on a cammed motor because they already start out borderline-lean on a stock motor.

Here's how to fix it: First off here's the path the fuel takes in the idle circuits: It goes through the main jets (which are NOT a restriction for the small amount of fuel the idle circuit needs) and into the main fuel wells. From there it goes up through the idle metering orifice. This is basically a vertical "straw" that has a tiny restriction at its tip. This restriction is typically ~.030". It serves basically the same purpose as the main jet, but on a much smaller scale.

OK, from there it goes up to the top of the "straw". There it makes a 90 degree turn and goes about 1/4" horizontally. At that point it goes past the Idle Air Bleed that mixes the liquid fuel with a little air from the main bore area turning it into a frothy mixture that I call "cappuccino foam". The reason for this is that a foamy mixture atomizes much better in the intake manifold than liquid fuel does. OK, now the cappuccino foam makes another 90 degree turn and goes back down- to the idle and off idle discharge ports. But before it does it goes through one last restriction- the Idle Channel Restriction. This is the final calibration point in the system other than the idle mixture screws themselves.

All right, we're through the Idle Channel Restriction and headed straight down to the idle discharge ports. Before we get there we go past the off-idle discharge ports (vertical slits, really, not round holes) where we mix in a little more air and then finally we get down to the idle mixture screws and the idle discharge ports. I think we can figure out how things work from there.

How to modify stuff: The first place to begin modifications is at the idle discharge ports. These are the round holes that the tip of the idle mixture screws go through. Since the idle mixture screws are tapered you get more or less mixture flowing through them as you screw them in or out. However, on a lot of QJets (almost all of them) the actual discharge port is considerably smaller than the mixture screw diameter. This is done purely so that some bonehead mechanic can't back the mixture screws far enough out to over-richen the mixture by very much. Again, all hail King Emissions!

How to fix: Take out the mixture screw. Measure it's maximum diameter with a micrometer or vernier caliper. Pick a drill bit that is .030-inch smaller than that and drill those suckers out! But first, take the carb off the engine (lets not get TOO lazy now), take the Throttle Body portion of the carb, the bottom part that has the throttle plates in it off the main body and drill the idle discharge port right through where the mixture needle screws in. Since the original hole is already there it acts like a pilot hole and messing things up while drilling is not gonna happen unless you're smoking crack at the time. Obviously, you don't want to leave any metal chips in there so its best to clean everything out before putting it back together and onto the engine.

One little snag you might run into (you knew there was going to be one, right?). Some carbs, and I do mean SOME, have yet another idle mixture restriction JUST BEFORE the mixture gets to the idle mixture screws. You have to take the throttle body part of the carb off the main body to see this restriction, so its best to check for this at the same time you are drilling out the idle discharge ports. It is in the throttle body casting and is in the vertical "down channel" just before the mix get to the idle mixture screw (you can see the threads of the idle mixture screw from the side through it). If this area is wide open, you're fine. If it has a restriction in it that is ANY SMALLER than your new idle discharge port (use the drill bit to check it) then you should drill this out ALSO.

All righty, just getting warmed up now.....

Drop the carb back on the engine and adjust the idle- making sure to keep the mixture screws and EVEN number of turns out each- not 1 1/2 turn on one and 3 on the other- EVEN! OK, Any better? Can you back the mixture screws out far enough to make the engine slow down from an over-rich idle mix? Do you have LESS THAN 4 turns out on the mixture screws to get an acceptable idle? NO? Ok, then you need to go farther.

The next mod will richen ALL the idle circuits up quite a bit- BOTH the curb idle and the off-idle circuits. It also helps cure an off-idle "sag" common to heavily cammed motors with big-runner intake manifolds. Remember the QJet was lean EVERYWHERE in its calibration and that includes the off idle circuits!

Alright. Take the air horn off the top of the main body of the carb- 11 screws, the choke plate pushrod, and the accelerator pump arm must be disconnected. Now before you mess with anything lay the original gasket back onto the main body of the carb (if it isn't too torn up- then use the replacement) but make sure you have it on right, not flipped upside-down. Its not a perfect mirror image but close enough to fool you sometimes. There are two small "kidney bean" shaped openings in the gasket sitting right next to each primary bore. At either end of the "kidney bean" is a little hole. These are the idle metering orifice (the "straw") and the Idle Channel Restriction that I mentioned about 5 million words ago.

You're gonna drill out the Idle Channel Restriction a few thousandths. Trouble is...knowing which is which. The Idle Channel Restrictions are the holes that are slightly farther apart and farther towards the front of the carb (nearer the fuel inlet), one per side.

Now you're gonna need some drills you probably don't have. You need what they call a "numbered drill set". This is a set of drills that are small and very closely spaced to each other. You can buy them from JC Whitney or order them from Sears. $30. In a pinch just pull out your 1/16" drill bit (.063")- its gonna make everything VERY fat, but for a non-emissions high performance, weekend-warrior motor its better to go too fat than too lean. If its a street motor and you expect ANY economy from the thing then buy the drill bits and do it right. Remember your street car runs on the idle circuits 95% of the time!

Use the drill bits to determine the size of your stock idle channel restriction by trying to put them though the restriction on on a go/no-go basis. Its probably gonna be about .045-.050". Pick a drill that is about 4-5 thousandths bigger than this and drill it out. The restriction itself is about 1/4" long. Try to drill straight down- use a drill press if you have access to one.

Why not drill out the idle metering orifice instead of the idle channel restriction? Because its about 2 1/2" down at the tip of the "straw" and no drill bit that small will reach that far. Otherwise, it would have exactly the same effect as drilling out the idle channel restriction.

Because of the metal filings generated its a good idea (all right- its REQUIRED) to pop off the throttle body (lower) part of the carb afterwords and clean out the channel and the idle discharge ports thoroughly before reassembling and putting back on the car.

After this mod you should notice that the mixture screws don't need to be nearly as many turns out to get an acceptable idle. Stock they usually need about 4-5 turns. After this you should be down to about 3. Also, the transitions (off-idle circuits) will be richer and many engines will feel much more willing to rev on light throttle after this modification.