Modify the Throttle Shaft
There's another easy trick that can help you squeeze some more airflow out of the 2G carburetors. The throttle shaft, like you can see in Photo 7, is what's called a sandwich shaft. In other words, there are two slabs to the shaft and the throttle blade sits in between them.

What you can do is cut off the top of the shaft and then mount the blade just to the bottom half of the shaft. And make sure to use a flat-head screw instead of the tapered screw like you see in the photo. That modification is usually good for an additional 10 cfm.

High Speed Air Bleeds
If you will look back to Photo 4, notice the two small holes on either side of the center mounting screw in the two top clusters. That is the high speed air bleed, and that is another thing you can use to tune the fuel curve besides the jets and the emulsion system.

The high-speed air bleed can be used to fine-tune the last quarter of the fuel curve. For example, let's say the motor looks good in the midrange from 4,000 to 6,000 rpm. The fuel curve is nice and on the dyno it's showing a 5.0 brake specific fuel consumption (BSFC) and 12.9 to 13 on the air/fuel ratio. But after six-grand, it starts going rich and you get mid fives on the BSFC and low twelves on the air/fuel. What you can do is open up the high-speed air bleed hole without changing the jet, and that will lean out the top of the fuel curve without ruining your midrange.

Again, this adjustment means taking a drill to the carburetor, so take it easy and be careful not to cut out too much at once.

Now look at the cluster at the bottom of that photo. You will see that the high-speed air bleed holes appear to be missing. They aren't, instead they are actually in the face of that area instead of on top. In other words, they are 90 degrees opposed to where the air bleed holes are on the other two. Having the high-speed air bleed holes on top works better, so you either want to avoid using one with the holes in the face or modify them. We actually drill out new holes into the top and block off the holes in the face with lead rather than use them that way.

Also, take a close look at the cluster on the top right. There are two holes that you can see in the nodules that stick out from the face area. You can also see them a little bit on the bottom one as well. Those are the accelerator pump discharge nozzles.

The discharge nozzles are also an area you can tune by opening up the holes. You might want to do this if you notice you are getting a stumble when coming up off the corner. That's essentially the equivalent of going to a bigger squirter on a Holley carburetor.

But to be honest with you, that's not usually necessary on a 2G because it's such a small, restrictive carburetor and is almost always maxed out on the racetrack. You don't need a lot of accelerator pump on the track because there is so much air flowing through the carburetor and you keep the engine at a high rpm on the track, so the main circuit gets activated very quickly. This means that there is usually little to no need for any pump shot once you are actually on the racetrack, because the air speed through the carburetor is so high since it's acting like a restrictor on the engine.

What you are trying to do when you are forced by the rules to run an undersized carburetor is deal with a situation where the air moving through the carburetor is at a far greater speed than the carb was ever designed to handle. When you are dealing with a four-barrel you are trying to encourage each circuit to respond more quickly and do its job better, but with the 2G you wind up trying to kill off some of the circuit's function because it's in a hyper mode, so to speak. The air speed is just too fast, so when working with these carbs, keep in mind that you are actually trying to slow things down in order to increase the quality of the state of the fuel when it gets to the engine.

SOURCE
Sean Murphy Inductions
N.A
AK
714-843-9169
www.smicarburetor.com