 An optimum air/fuel curve...  An optimum air/fuel curve is what racers want out of their stock two-barrel carburetors. The accelerator pump discharge nozzles aim fuel at the booster venturi to help atomize it. |
 A dial indicator was used...  A dial indicator was used to make sure the boosters were level and at the same height in each venturi. |
 McLendon resurfaces the area...  McLendon resurfaces the area between the metering block and the carburetor to ensure a good seal. He also removed a little material from the accelerator pump passage to prevent any clearance problems due to the resurfacing. |
 The air passages in the throttle...  The air passages in the throttle butterflies were drilled out so the blades could be closed and still allow the correct amount of air to pass at idle. |
 After installing and aligning...  After installing and aligning the boosters, McLendon doesnt take any chances that they will move. He mixed up some epoxy and secured each one. The carburetors base was held at an angle to allow the epoxy to get behind the mounting area and to form a smooth angled surface. |
 McLendon suggests a small...  McLendon suggests a small mark with a permanent marker on the float adjuster nut. He says, It always gives you a point to go back to start over if necessary. |
 A good way to set up the float...  A good way to set up the float is to draw an imaginary line going from mounting hole (where screwdriver is pointing) to mounting hole, and adjust the float so the bottom is at that line. The float can be fine-tuned after it is installed on the engine. |
 Steel gallery plugs replaced...  Steel gallery plugs replaced all the vacuum holes that were no longer needed, and at the end of the shortened throttle rod. As a precaution, McLendon added a spot-weld to the throttle lever where it is press-fitted to the throttle rod. A press-on piece always has a chance to slide, resulting in a carburetor that doesnt respond, he says. A spot-weld is great insurance. He also shortened the throttle shaft where the choke assembly was, and plugged the exit hole with a steel galley plug to prevent air leaks. |
 The stock (left side) vent...  The stock (left side) vent baffle (arrow) was changed to a larger one (right side), which reduces the chance of fuel flowing up the vent tube into the venturi. |
 Metering blocks with three...  Metering blocks with three different emulsion air bleed hole setups. The one on the right is stock and similar to the one we used for this story. The middle one is a typical Late-Model setup and the one on the left was used on a Busch car. |
 This needle valve is located...  This needle valve is located at the outlet for the accelerator pump. The jet on the right is a stock valve and the one on the left is used because its heavier. Note how the mounting screw is drilled out so the enlarged valve will slide into it. As the engine rpm increases, the engine vibration can cause this valve to bounce, allowing fuel to be siphoned into the venturi. The heavier valve prevents this from happening. |
How do you turn your out-of-the-box carburetor into a Street Stock performer so you can edge out the competition? Increasingly, racing classes are limited to a certain type of carburetor with only certain modifications allowed. The art of modifying carburetors within the rules is not trivial, but it is straightforward. We went to Scotty McLendon of McLendon Carburetors for some help in taking a stock production carb out of the box and making it a Street Stock racing piece.
The idea in any carburetor modification is to build an optimum air/fuel delivery curve throughout the entire rpm range. As the engines rpm increase, so does the demand for more fuel. A carburetor that delivers too much (running rich) or too little (lean) fuel can steal power from your engine, giving you slower lap times and potentially damaging your equipment. There are several modifications that can be made to keep the air/fuel curve at its best power at all rpm ranges.
Fit for Racing
McLendon helped us turn an out-of-the-box, 500-cfm Holley two-barrel carburetor (PN0-4412C) into a carburetor fit for racing. This particular carburetor is one you would see bolted to a typical Chevy 350 engine on a local short track.
He started by disassembling the carburetor and doing away with nonrace items like the choke linkage, choke blade and extra vacuum outlets. Removing the vacuum outlet tubes and replacing them with steel gallery plugs is very important because it eliminates any chance of air leakage that could change the air/fuel mixture, he says. The conventional rubber boots that slide over the vacuum tubes are not used because they can deteriorate, creating future air leaks.
Transfer Slot Checking
With the unnecessary items out of the way, McLendon moved to the carburetor body. At the bottom of the venturis, he started with the transfer slots, which are small openings that supply fuel to the carburetor during idle and the early stages of acceleration until the vacuum builds and the main fuel circuit takes over. There is a transfer slot for each venturi. Each of these slots needs to be the same length and 0.025 inch wide. This is to ensure equal performance from each venturi. To check and make any adjustments, the baseplate, butterflies and throttle shaft were removed. McLendon had a file made just to fit these slots. Each slot also needs to start and end in the same spot with 0.020 inch showing below the throttle plates. This amount may or may not need to be adjusted when the carburetor is bolted onto the engine. The transfer slot opening is not adjustable like a needle valve is, so its important that it is properly sized.
While the throttle shaft was removed, McLendon shortened the shaft end where the choke assembly was attached. With the shaft shorter, it will no longer exit the side of the carburetor, so a gallery plug was inserted to remove the chance that air and dirt might contaminate the air/fuel mixture.
Booster Balancing
Next came the venturi boosters. The boosters create a high vacuum source for the main jets to pull fuel from the fuel bowl. This is one of the most commonly checked areas on the carburetor when youre at the track. Here, McLendon made sure that each one was parallel to the venturi throat and at the same height.
Making the booster parallel to the venturi throat directs the air straight down into the carburetor and not at an angle. If the booster is at an angle, the air velocity will be disturbed and its velocity slowed. Slower air velocity means less fuel delivery, so McLendon spent a little extra time making sure the alignment is precise as possible. He adjusted the height of both boosters to be within 0.002 inch of each other, which also keeps the venturis equal in perfor-mance. During reassembly, note that the throttle butterflys mounting holes are not centered. An easy way to remember the proper way to reattach it is to bolt it down with the imprinted numbers facing the intake manifold.
Air-Bleed Sizing
The air bleeds were the next modification. There are two different types of air bleed in the carburetoran idle air bleed and a high-speed or high-rpm air bleed. Their function is to help suck up the fuel from the main well and to start atomizing the fuel. In the case of this carburetor, the idle air bleed was increased to 0.073 inch from the stock size of 0.070. The high-speed side was also increased from 0.026 to 0.029. The size of the air bleeds depends on your application.
The metering block had a few alterations, starting with the emulsion air bleed holes. These holes are designed to let air into the mixture as the vacuum decreases or engine rpm increases. Additional air in the fuel mixture as the rpm increases helps stabilize the fuel curve, preventing it from getting too rich or too lean. On this stock carburetor there are two emulsion holes, one near the top and one near the bottom of the emulsion channel in the metering block. The type of racing you do will dictate how you change the diameter and location of the emulsion holes.
With this carburetor being altered for a Street Stock, the number and diameter of the holes stayed the same. The factory holes range anywhere from 0.027 to 0.029 inch. These diameters are decreased in size as the number of emulsion holes in the emulsion channel is increased. In some applications there are enough emulsion holesup to eightin the metering block to control the fuel mixture in smaller increments as the engine increases in speed.
Power Valve Matching
Also in the metering block, the power valve was removed and the power valve jets were checked for size (the No. 73 jets measure 0.079 inch). The power valvespring determines when it will open, and its stock setting is 2.5 inches of vacuum.
It can be set as high as 10 inches of vacuum. No alterations were made to the stock vacuum power valve setting or to the power valve jets for our application.
There is a common misconception on how a power valve works. The valve itself doesnt affect how much fuel gets to the engine; it only activates when engine load increases and additional fuel is needed. Within the power valve circuit is one jet per venturi. This jet size is the determining factor on how much fuel is added to the normal mixture when the power valve opens.
Idle Feed Relocation
The next alteration to the metering block is the idle feed restriction holes. These holes are drilled out and relocated to the opposite end of the channel, keeping their stock 0.036-inch size. This reduces the travel distance of the fuel, giving better throttle response off-idle and when coming off a turn. As most Street Stock races are short in duration, so restarts and power off the corners are of prime importance. The main fuel jets start out at 0.072-0.073 inch from the factory. The jets control the main supply of fuel at speed. Since the carburetor we worked on was meant for a Street Stock, the jet sizes stayed the same. McLendon simply checked to see that both were the stock size and equal.
Assembly
After modifying the carburetor, McLendon then turned to the task of assembling the pieces. The first change was to the cam that actuates the accelerator pump. He took out the stock burgundy cam and replaced it with a smaller orange one. The stock cam pushed the accelerator pump arm where it adds too much fuel, he says. This can flood the carburetor and reduce the throttle response. Another option is to change the size of the pump itself depending on the volume you need for your type of racing. Remember, more fuel is just thatmore fueland more is not necessarily better. It may reduce performance.
McLendon also changed out the arm that rides on the accelerator pump cam for a smaller, lighter one to save weight. The stock arm weighs more than twice its replacement. He then adjusted it so it just makes contact with the accelerator pump at about 0.015 inch at WOT. If there is a larger gap between the two, a hesitation will occur when you step on the gas pedal.
Once the carburetor was completely reassembled, McLendon bolted it to a 350 Chevy test engine to check for leaks and make final tuning adjustments. Both idle-mixture screws were turned out one full turn for a starting position and the engine was cranked up. With the engine running, McLendon set the idle speed by adjusting the throttle blade angle to support 900 rpm. Then he could adjust the idle-mixture screws. In our case, the idle-mixture screws had no effect on the engine, even with both of them closed completely. The throttle blades were not closing enough and exposing too much of the transfer slot on the manifold side of the carb. This allows too much fuel into the engine and wont allow it to idle down.
To correct this problem, McLendon removed the carburetor from the engine and unbolted the baseplate. He clamped the baseplate in a vise and drilled out their stock-size air holes in the butterflies from 0.093 to 0.149 inch. This increased the airflow going into the engine at idle, and allowed him to mechanically adjust the butterflies with the idle adjustment screw to close farther, reducing the amount of fuel available from the transfer slots. Now the engine could be properly adjusted at idle using the idle-mixture screws.
With these tuning tips, your carburetor should no longer be an average run-of-the-mill air/fuel supplier. It should now mix air and fuel efficiently, giving your engine the power to edge out the competition.