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
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