The much more rare 840cfm...
The much more rare 840cfm Quadrajet carb.
Until throttle body fuel injection started taking over in the 1980s, if you had a General Motors product with a V-8 engine, you could just about guarantee a Quadrajet manufactured by Rochester would be sitting on top of the intake manifold.
And while Quadrajet-style carburetors aren't nearly as popular these days-especially when it comes to stock car racing-as the iconic Holley-style four-barrels, you'll still find them on both the street and the racetrack. This is particularly true in the Pure Stock and some IMCA Modified classes that require racers run the same carburetor that originally came installed on the engine. Of course, racing with a Rochester Quadrajet isn't exactly a death sentence to poor performance and on-track frustration. When properly prepared and tuned correctly, these carburetors actually are very good performers. The trouble is working with a Quadrajet is becoming a bit of a lost art.
Sean Murphy of SMI (Sean Murphy Induction) is one of the few carburetor builders and tuners around who is still trying to advance the art of racing with a Quadrajet. We recently spent some time with him and he graciously shared several tips for getting the most out of these carburetors. What follows isn't your typical rebuild story-if you want that, there are plenty of books out there-instead, Murphy gave us the real lowdown on what it takes to turn these carbs, which can be rather pedestrian in stock form, into real screamers. The tips were so good we decided to pass them right along to you in his own words.
A stock 750cfm Rochester Quadrajet...
A stock 750cfm Rochester Quadrajet carb.
An 800cfm Q-Jet.
How Big is It?
There are basically three different sizes Rochester produced for the Quadrajets. Image 1 is a stock 750cfm carb. If you look at Image 2, that's an 800cfm. The main difference with these two is the bump on the venturi wall is significantly larger than the others. That bump was to enhance the venturi effect, which increases airspeed across the booster and, in turn, increases the signal at the booster. But it comes at the cost of decreased overall flow.
Image 3 is the 840cfm carburetor. It's easy to identify because it doesn't have the secondary booster ring. That's how it gets such a larger airflow. These carbs are more rare because the only place you'll find them is on '71 and '72 high-output applications on Pontiacs outfitted with 455 engines.
By removing that bump on the venturi wall, you can really increase the amount of cfms these carburetors will flow, but you have to be careful. You don't want to get too happy with the Dremel and mess up the shape of the venturi. As an example, you can look at Image 4 to see a before and after. On the left is a venturi that we've modified for maximum flow, while on the right the venturi is still the way it came from the factory.
When we go in and modify these carburetors, we can get as much as 850 cfm out of the 750. And on the 800 we can get it all the way up to 890 cfm, but it really depends on what your engine needs. You can over-do it with the carburetor size. Remember that many of these engines are limited by the rules, so if you get too large you will lose responsiveness off the corner. In circle track applications the most popular carburetor to start with is the 750.
We also look at the quality of the airflow through the carburetor. In the stock carburetor (Image 5), you can see casting flash in the secondary venturis. To get a little more airflow and also decrease the turbulence as the air moves through that area, we will polish the secondaries (Image 6). It doesn't look like much, but by decreasing the turbulence you can increase the overall airflow.
On the left is a venturi that...
On the left is a venturi that we've modified for maximum flow, while on the right the venturi is still the way it came from the factory.
In the stock carburetor you...
In the stock carburetor you can see casting flash in the secondary venturis.
To get a little more airflow...
To get a little more airflow and also decrease the turbulence as the air moves through that area, we will polish the secondaries.
The fuel comes through the...
The fuel comes through the secondary main wells then comes back up and through the jet. You can see how restrictive this is.
We will work the secondary...
We will work the secondary main wells in order to improve fuel flow through them.
Here, you can see the different...
Here, you can see the different springs that are available for you to tune with.
Secondary Main Wells
The primary power piston system...
The primary power piston system is arranged inside the carburetor and controls the primary fuel circuit.
The fuel comes through the secondary main wells then comes back up and through the jet. You can see in Image 7 how restrictive this is. We will work the secondary main wells (Image 8) in order to improve fuel flow through them. If you have more than 500 hp, then this modification is necessary in order to sustain enough fuel flow. This is because the secondaries actually flow about 75 percent of your fuel at wide open throttle. So it's critical in high-horsepower applications that you spend a little time working this area because no matter what metering rod you put in the carburetor you just won't be able to get it rich enough.
The Primary Power Piston System
Image 9 (following page) shows how the primary power piston system is arranged inside the carburetor and controls the primary fuel circuit. As you can see, this setup gives you lots of different tuning options. You've got a jet, you've got a metering rod, you have the power piston spring, and you can also move your metering rods up and down inside the jets.
Power Piston Springs
Here, you can see the different...
Here, you can see the different springs that are available for you to tune with.
One area where you can tune the primary power piston system is the spring. In Image 10 you can see the different springs that are available for you to tune with. In a circle track application, you are kind of limited when it comes to tuning with these springs because you will have a big cam with low vacuum. During restarts, if you don't have a spring with low enough pressure you can end up with a rich condition while you are running around, then when you hit the gas on the restart the motor is all loaded up and you lose a couple positions because you're waiting for it to clear out while your competition is going by you.
Unfortunately, GM has discontinued a lot of the parts for Quadrajet carburetors, including many of the springs. Edelbrock does sell a kit, but only a few of those springs are useful to the circle track guys. So what we usually end up doing is using that long spring that's on the left and trim coils off depending on the camshaft specs. That way we can custom tailor the spring to the application. This has required a lot of research before we could get this right so I wouldn't recommend it to anyone. But that is one option that is available.
Primary Metering Rods
Like the springs, in Image 11 you can see a variety of the different primary metering rods. You should be able to see that their styles are different on the lengths and the taper to the end of the rod. These rods are identified by a number and a letter. For example, you could have a 45K or a 50M. The number indicates the diameter in thousandths of an inch across the thick part of the rod. And then the letter designates the style of rod, whether it's a straight taper at the end or a stepped taper. There are 10 or 12 different styles of rods, so you can see that when you combine that with the number of different sizes available that turns into quite a number of different tuning options. All those tuning options mean the Rochester carburetor can be tuned very accurately, but it can also be a bit intimidating if you go into it blind.
Idle Down Tube
Here (Image 12) we are drilling out the idle down tube, and that is your jet for the idle circuit. It is a fixed tube that's about an inch and a half long, so you need a special drill bit in order to modify that. But basically, the bigger the camshaft, the bigger that orifice has to be in order to send enough fuel to the engine to overcome the inefficiency of the camshaft at low rpm. Also, as you open up the idle down tube you will also need to enlarge the channel restriction orifice (Image 13) as well because it shares fuel.
We are drilling out the idle...
We are drilling out the idle down tube, and that is your jet for the idle circuit.
As you open up the idle down...
As you open up the idle down tube, you will also need to enlarge the channel restriction orifice as well because it shares fuel.
The primary throttle bore...
The primary throttle bore looking from the bottom of the carburetor.
Idle and the Primary Throttle Bore
Earlier Quadrajet setups had...
Earlier Quadrajet setups had exposed mixture screws.
Image 14 is a photo of the primary throttle bore looking from the bottom of the carburetor. The small hole in the center of the picture with the small hole below it is the idle discharge port. And again, the bigger the camshaft, the bigger that hole needs to be. But you can only go to certain maximum sizes depending on the type of mixture screw you have on the other side of that hole. We'll get into that a little bit later.
In that same photo, the hole that's in the 11 o'clock position is the idle air bypass. That's essentially a metered vacuum link through the carburetor and it essentially does the same thing as drilling holes in the butterflies in a Holley carburetor. It allows air to bypass into the engine without opening the throttle plates any more, so you don't overexpose your transition slot while the engine is at idle speed. The idle air bypass can be drilled out to a maximum of 0.140-inch to control idle speed. The position of the butterfly you see here is the optimal position in order to keep the right amount of the transfer slot exposed. If you can't get there by drilling out the idle air bypass, you may also have to drill a hole in the butterflies as well.
Like a lot of other adjustments, when it comes to drilling holes in your carburetor, this is all a trial-and-error process. Just remember to take it easy and slow. It's easy to drill a hole a little bit larger, but it's a lot harder to put that material back once you've drilled it too big.
I mentioned mixture screws earlier. Quadrajets were made with two different types of mixture screws. The earlier setup had exposed mixture screws (Image 15) which are easy to see and get to. Later, the mixture screws were adjusted at the factory and sealed with a plug. In Image 16 you can see a carburetor with sealed mixture screws that we've modified so that they can be accessed by a carburetor tuner.
You can only open up the idle discharge port to a certain size depending on which mixture screw your carburetor is equipped with. With an exposed mixture screw you can bring that hole up to 0.110-inch, but with a sealed mixture screw the maximum size is limited to about 0.093. You can't convert sealed mixture screws to the exposed style because of the way the threads are set up. The sealed mixture screws use a fine pitch metric thread. They did that at the factory to allow for a more accurate adjustment. But because the thread pitch is different, you can't just drill out the threads and tap that hole with the threads for an exposed mixture screw. There's just not enough meat in there to go in and do that. Believe me, we've tried.
Accelerator Pump Tuning
In Image 17, we can see the different lengths of the springs and the different assemblies for the accelerator pump. The shorter the stem on the accelerator pump, the more volume of fuel you will have. It's almost like using a 30cc or 50cc pump on a Holley. Because the fuel capacity is stored underneath it, and by using a shorter pump, you increase the size of the cavity. Then when you hit the throttle and the pump moves down into the cavity, the shorter stem will move more fuel than the longer one.
The spring is called the delayer spring. The point of this is you can't always move fuel as fast as you can mash the gas pedal. Since you can't compress a liquid, it's the job of the delayer spring to prevent bending a linkage or damaging anything. Then it will discharge the fuel as the spring pushes back on the plunger.
What you want to watch out for is that you don't run into a coil bind situation by running the wrong spring with the wrong pump. So the spring we most often use is the one all the way on the left because it has the fewest amount of coils and is the shortest. It also has the thickest coils, which helps with two things.
Later designs feature difficult-to-access...
Later designs feature difficult-to-access sealed mixture screws. Here you can see how SMI has modified the screws so that they can be accessed by a carburetor tuner.
Here, we can see the different...
Here, we can see the different lengths of the springs and the different assemblies for the accelerator pump.
Here is a selection of hangers...
Here is a selection of hangers for the metering rods.
First, what can happen if the spring and pump are poorly matched is the spring will not have enough tension and will compress before the pump starts moving down into the cavity. Then you get a hesitation because the pump won't start pumping until the spring has bottomed out. So the spring has to be strong enough to ensure the pump starts moving first before the spring starts compressing. That's why the spring we use the most has thick coils to be strong enough, but not so many coils that it will go into coil bind.
Metering Rod Hangers
Image 18 shows a selection of hangers for the metering rods. As we move down the picture, the hole location in the hangers move down as well. The hangers are sorted by a letter that's stamped on them, and basically, the higher the hole, the higher the letter. It starts at "B" and goes all the way down to "Y" and each moves the hole in an increment of 0.005-inch.
One way to take advantage of this is, say when the secondaries begin to open you have an overly rich condition. You can use the hanger to locate the metering rod down deeper into the jet, and that will lean out the initial fuel curve as the secondaries begin to open. So, combine that with the different styles of metering rods with the different tapers and you can just about come up with any type of fuel curve that you need.
Well, there you have it. Take these tips to the shop and start really tuning on that carb and we'll see you in Victory Lane!