Earlier Quadrajet setups had exposed mixture screws.
Idle and the Primary Throttle Bore
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 sealed mixture screws. Here you can see how SMI
Here, we can see the different lengths of the springs and the different assemblies for the
Here is a selection of hangers for the metering rods.