The primary power piston system is arranged inside the carburetor and controls the primary
Secondary Main Wells
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.
Here, you can see the different springs that are available for you to tune with.
Power Piston Springs
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 down tube, and that is your jet for the idle circuit.
As you open up the idle down tube, you will also need to enlarge the channel restriction o
The primary throttle bore looking from the bottom of the carburetor.