Main Body Sizes
There are two basic sizes that are used in circle track racing. The two sizes came from the factory, and you can still find either fairly easily. The smaller carburetor is on the left in Photo 2, and you can easily tell by looking at the size of the center divider between the venturis. That unit on the left is what we refer to as a 390-cfm, and the venturis are 1 1/4 inches in diameter. The one on the right is for a 500-cfm carb and the venturis are larger at 1 3/8 inches.
The problem is, the 1 3/8-inch venturis are better for racing applications because they flow more air, but they are also going to be difficult to find. So we've figured out how to machine the smaller ones out to the 1 3/8 size because they were the more popular size and are still easier to find.
If you are hunting for a 2G carburetor and want to figure out which one of the two you are looking at, there is actually a very easy method. Rochester produced the 2G with a ton of different part numbers, so it can be really confusing to try to check the part number on some random unit and try to figure out what you are looking at.
Instead, you can take a 1-inch open-end wrench and use that. On the 1 3/8 unit the open end of the wrench will just barely fit over the divider between the two venturis. But on the 1 1/4 unit, the divider is larger and the wrench won't fit over it. It's a quick and easy go/no-go tool to figure out what you've got.
Emulsion Hole Patterns
Photo 3 shows the different types of emulsion hole patterns that are available. If you look at the big tubes in the brass tubes in the middle of the clusters, you can see that there are both different size holes and different numbers of holes. Unlike the size of the venturis, where you almost always want the larger size, here one style isn't necessarily better than another. It's more of a situation where some fit certain applications better than others. You just have to figure out which work best for what you are trying to do.
Occasionally, we'll modify these hole patterns further to get what we're looking for. Unfortunately, this is one area where I can't tell you one way that works best. It requires a lot of experience to learn and understand what works best for each situation, and the differences can be small.
The emulsion holes aren't a tuning tool. In other words, you won't be changing them out depending on what track you are running or if conditions change. What we usually do is pick the best one for the application and then set it up so that the racer only has to change the jet to get the correct fuel curve.
If you look at the cluster on the bottom left, you will see that it has four emulsion holes, and the bottom two are larger than the top two. Sometimes we will enlarge all four holes. Sometimes we will only enlarge a couple. And sometimes we will add a couple more holes and enlarge those. So it's basically the number and size of the holes that we will work with to custom tune the carburetor to the application.
The smaller tubes on the outside edges of the clusters are what we call the idle pickup tube. Those are basically the jets for the idle circuit. And you can see in all but one they have a little extended tip on the bottom that's a smaller diameter than the rest of the tube. That is the area where the restriction is, so that's basically the jet orifice, if you will.
As a general rule, the larger the cam, the larger that tube needs to be. And you can see on the one on the bottom righthand side, those extensions are gone. That's because it's going to be used in a car with a very aggressive camshaft. With a real big cam, it wants all the fuel you can get down to it, so sometimes you just have to open up that tube as much as you can.