Photography Courtesy of SMI
When you think about it, it's kind of funny that motor racing is one of the last places you will find a two-barrel carburetor. Meanwhile, you will have to look pretty long and hard to find a race car driver who actually loves racing a two-barrel.
Honestly, the only reason you will find a two-barrel carburetor on a race car is because the rule book requires it. A four-barrel carb is almost universally better at providing the right combination of good throttle response on the track while also providing plenty of air to the engine for power. Two-barrel carburetors are still around in racing mostly because sanctioning bodies continue to use them as a restrictor plate of sorts. By requiring a two-barrel on a Late Model Stock V-8 or some similar class, the carburetor limits the amount of air the engine can ingest, which in turn, limits the horsepower it can make.
So obviously, any legal improvements you can make to the carburetor will result in significant power gains. The same goes for making sure your two-barrel is properly tuned. And there is plenty that can be done to make a Holley two-barrel flow more air more efficiency than it ever could in stock form. By the way, although a 500-cfm two-barrel normally used with Mini Stock race cars running four-cylinder engines (mostly Ford's 2,300cc from Mustangs, Pintos, and Ranger pickups) can provide about all the air and fuel these four-bangers can handle, winning performance is also dependent upon proper carb tuning.
An entire industry has sprung up around building and tuning two-barrel carburetors for racing, and while many of these carb specialists can work miracles with a two-barrel it doesn't mean that you have to depend on them for everything. There is actually a lot you can do yourself to help improve a two-barrel carb. And if you are trying to race on a limited budget or build a Mini Stock as your son or daughter's first race car, you may not need a high-end, professionally built carburetor. Instead, doing a little tuning work yourself can significantly improve your performance over stock without costing you much at all.
With that in mind, we turned to Sean Murphy of Sean Murphy Induction, one of the most successful carb building and tuning shops in the nation. We've worked with Sean and SMI in previous issues of Circle Track on racing Rochester/Quadrajet carburetors and were greatly impressed both with the depth of Sean's knowledge and his willingness to share it. (If you want to see those previous two articles, just go to www.circletrack.com and type "Rochester" or "Sean Murphy" in the search box. You'll be glad you did.) So obviously, when we wanted to learn more about building and tuning the venerable Holley two-barrel, we went back to Murphy and he practically buried us under an avalanche of great information.
The following is just some of what SMI gets into when it preps a Holley two-barrel carb. This definitely isn't everything they do, but it should be enough for anyone to improve their own racing program by ensuring their two-barrel carb is properly prepared for the task at hand.
We went back to Murphy and he practically buried us under an avalanche of great information
 Unlike many carb specialists that rarely get any farther than a flow bench, SMI performs significant testing on its carburetors in real-world situations. SMI's Sean Murphy says the Holley 4412 was originally designed for the Ford FE big-block typically installed on 3/4 and one-ton trucks. So you can see why it is quite out of its element on a high-revving race car. Still, SMI has been able to make more than 530 hp with these carburetors.
 One of the keys to properly tuning any carburetor is getting the metering block right. Not all metering blocks will work in every situation. As an example, SMI provided this photo of three different metering blocks. The top block is a billet aluminum unit designed to work in racing applications burning alcohol. The middle block is a stock unit, and the bottom is the improved metering block from an HP carb.
 A circle track racing carburetor must perform well at idle, full throttle and everywhere in between. It doesn't matter how much peak power the carburetor makes if it causes the engine to stumble every time the driver gets on the throttle coming out of a turn. This photo shows the butterflies with a stock hole in the left plate and a larger hole SMI has drilled through the right plate. The larger hole allows more air to bypass at idle, which is needed by a larger-than-stock racing cam. This keeps the throttle plate at the correct angle (closed) while still providing enough airflow through the hole to keep the engine idling at the correct rpm.
 At the top is a stock 50cc accelerator pump. Below that is a 30cc pump. The larger 50cc pump is undesirable in all cases with the exception of alcohol carbs and some low rpm four-cylinder engines. On a V-8 race engine the 50cc accelerator pump will cause an overly rich condition when introducing throttle off the corner. This will result in the car feeling lazy. The reason is because the airspeed is too high due to the carb being undersized for the application (going back to that whole restrictor plate thing). Therefore, the main fuel circuit, or booster, is almost always active and delivering fuel. So it is unnecessary for the accelerator pump to deliver the same amount of additional fuel that would be needed with a properly sized carburetor.
 That's a stock float on the left and a tapered float on the right. As the car enters the corner, g-forces push the fuel in the float bowl to the right side, which picks up that float. When the float moves up it closes the needle and seat, cutting off fuel delivery while the car is in the corner. When the g-forces are sustained like in a long corner, this can create a situation where the fuel level in the bowl is too low when the car exits the corner and the driver opens up the throttle. A lean stumble can occur as the fuel system attempts to "catch up" and fill the bowl once again. The tapered float can help keep this from happening by not allowing fuel slosh to get under it and push it up if the fuel bowl isn't actually full.
 A carburetor can run great, but if it gets you thrown out at the tech shed, it isn't much good to you. Because of this, Murphy says SMI goes to great lengths to make sure its carbs push the limits as far as possible without going over the line toward the dreaded "illegal." This is a go/no go step gauge they've created to make sure the carburetor venturiis are as large as possible while still being legal by the gauges used by your friendly neighborhood tech man. That ridge is pretty small.