The Cluster Center Hole
In Photo 4 we are still looking at the clusters, just at a different angle. Now we're showing the different center hole sizes that can be found in the cluster. The one on the top right is what everybody considers the most desirable. It has a 1/4-inch diameter booster which is good for total airflow but not necessarily as good for atomization of the fuel or even the signal strength in the venturi. So you may have to work in other areas of the carb to overcome those problems. Also, that hole size is by far not the most common, so it isn't always easy to find.
The center hole size that is the most common is in the cluster on the bottom of the photo. So, what we've done is figure out a way to modify that unit to the 1/4-inch hole because it simply is not worth the time and effort to go chasing after the other cluster. I'd say 90 percent of the 2Gs you will find will have a cluster like the one in the bottom of the photo.
Also notice the cluster in the top right. At the top there are two holes adjacent to and above the mounting screw holes. The one on the upper right has some small brass restrictors in those holes. That's the idle air bleed, and it will have an effect on your idle mixture as well. As we've talked about earlier, the down tube kind of acts like the jet, and that's the air bleed for that. The way it basically works is the idle tube comes up just to the left of that and then it mixes with the air for the idle bleed, and then it's transferred down through the idle discharge in the baseplate. So that's another tuning aid when using larger camshafts. You can alter the size of the air bleed to modify the fuel curve on the idle circuit.
The Channel Restriction Orifice
This one can be a little bit difficult to see, but it is still important. We're still working with the cluster as before. The fuel comes up the idle tube and then mixes with the air in the air bleed. And then in Photo 5, you can see the arrow pointing to the hole that it comes down. The problem is there's actually a restriction in there. It's a little brass restrictor, and that one is critical because it has a big effect on the transition slot. Basically, it meters the total amount of air and fuel mixed together that the idle circuit will get. That is called the channel restriction orifice. Generally, we will drill that restriction out to between 0.055 and 0.063 depending on the situation.
In this case, if you find that you've drilled it out too large it is possible to fix it. Just drill it on out further and insert a Holley air bleed.
By the way, while we are looking at the boosters in this photo, there is another booster design out there that's got a bit of a mythical reputation to it. That one's a bit famous because everyone wants one but so few have ever seen it. It's simply called the "Truck Booster" and it has really good signal strength and atomization properties. But it's very rare and very hard to find, it's really a piece of 2G trivia more than anything.
What you see in Photo 6 is something that we mainly do. That's a stock length booster on the left, and on the right is a booster that we've shortened and tapered. This modification can show some very significant gains in airflow, but you've got to check your rules to make sure it's legal or at least make sure you won't get caught. This is one of the best modifications you can make in terms of increasing performance because we're talking about gains in the range of 30 cfm.
And there's another trick you can do to help the airflow even more. There is a gasket that sits underneath that piece. You can stack a couple gaskets under there and that picks the booster up out of the venturi which also increases airflow. I don't know if I should give that one away because I haven't seen any of our competitors doing that one yet, but there you go.