Although you may have a hard time getting them to admit it, for many car builders, the carburetor is the most complicated and least understood part on a vehicle. Everyone understands the basics of its operation, but it is rare that you find someone who knows everything about how they work and, more importantly, what can be done correctly to improve its operation.
Nearly everything that is either in or on an engine (headers, camshaft, heads, intake manifold, so on) is analyzed by the carburetor in one degree or another, and each affects how the carb performs.
Some say that 80 percent of all carb problems are really ignition/timing problems (the duration of a cam dictates how much vacuum the engine produces, and controlling that vacuum is carb's responsibility), so anytime you change your intake manifold or cam, you need to recalibrate your carburetor from its previous settings.
But making carb changes also means dialing in the difference between the performance and the drivability of your ride. And, when there really is a problem with the carb (the most common is a dirty or stuck needle seat), most folks think a simple gasket change and cleaning will fix the problem. If they're lucky, that's true. But the guys at The Carb Shop (Ontario, CA) have a few decades of experience looking into what makes a carburetor work and are adept at improving what many of us would like to make a doorstop out of!
The Carb Shop (comprised chiefly of two brothers, Bob and George Vrbancic) offers five different levels of carb rebuilding, from simple machining of carb base plates and body surfaces (and yes, they can come uneven from a manufacturer) to all-out circuitry modifications and/or installing parts and pieces that have been developed after their many years dyno testing their product and techniques.
Bob and George will work on Holleys, Ouadra-Jets, and some other types of downdrafts, but they pass on working on Weber IDA, DCNF, DCOE sidedrafts, and SU-type of carbs. The cost for a street-driven carb rebuild is between $200 and $500, and the work will take a couple of weeks to complete. But why would you want to spend an additional $300 on a carburetor that might cost the same amount to replace? Some folks are attached to the carb they've got, but it's safe to say that no carb out of a box will perform the same as the ones from The Carb Shop.
When you have The Carb Shop go through your carb, they have you fill out a worksheet that gives them that particular carb's parameters. Besides the engine size, what type of usage the engine will see (percent of drag use versus street use), off-road or marine, launch rpm, nitrous or supercharger, header and collector size, the camshaft duration at .050 vacuum signal at idle, and even the altitude it will be driven in are all given consideration as to how the carb will work in its intended use.
They can also run the carb on the test mule motor on their own dyno, check the fuel curve, and determine peak performance numbers in a number of categories. You just can't get that type of perfection out of a box!
Gently pry the metering block off the carb body, careful not to break the alignment pins holding it in place. | 
After removing the gasket on both sides of the block, remove the pin from the vent whistle (the long, brown piece of plastic). It helps keep the gas from sloshing around in the bowl. | 
Remove the power valve as well as the main jet (found on the other side of the metering block) and also the air/fuel mixture screws from each side of the block. Do this work on both metering blocks. (Larger Holley carbs, such as a 750, have four air/fuel mixture screws in the block.) |
After removing the three screws that hold the choke assembly in place, remove the cotter pins that hold the assembly to the thermostat. There is a fast-idle lever behind that, which needs to be removed. | 
The secondary's accelerator pump lever is removed and checked for wear. | 
The pointer shows this carb's early-style lever clip used to secure the lever. Later models (right) have the clip on the outside. |
Removing the choke assembly at home isn't necessary but is done at The Carb Shop. Sometimes the butterfly brass screws strip and have to be removed with an easy-out type of tool. | 
Inside the primary intake, after the discharge needle screw is removed, a small pin will fall out (remember where you put this pin for later!). This pin is the accelerator-pump discharge needle. | 
The idle screw is removed, as is the link between the primary and secondary (and will be modified later). |
On early models (like this one), there are eight screws that hold the base plate to the carb body (later models use six). | 
Carefully knock all four boosters (which look like mini smoking pipes) out of the main body. | 
Early-style boosters are straight (not shown) but the new ones will be angled (right). The Carb Shop reams these, plus adds a step to create more turbulence (left). |
Using an ultra-fine wire wheel, remove the rest of the gasket material from the metering blocks and bowls. | 
All the parts are washed for 20 minutes at 129 degrees in a sonic tank, which removes the rest of the grime. | 
The butterfly screws are drilled out and the butterflys removed from the base plate. |
The vacuum piston is also removed from the thermostat housing. | 
The idle well plugs and the main well plugs (which act like freeze plugs) are drilled, then knocked out. | 
These parts just got bead-blasted. A die-chromate process will be added to the pot metal pieces (turning them back to their light-gold factory color). The base plate is made from aluminum, so it won't get the process. |
After the base plate is resurfaced to ensure flatness, all holes are deburred and rechamfered. | 
Permatex red light synthetic grease is used on three areas of the shaft for reinstallation. Permatex Threadlocker Red is used on the butterfly screws for installation. | 
The primary/secondary link is carefully bent in a vise and reinstalled (the bending allows the secondary to open fully). |
You can see how much warpage has occurred by looking at the edge of the accelerator pump cover (a common problem on the early-model carbs). The Carb Shop machines this part flat before reinstalling it and the accelerator pump. | 
The thermostat assembly is rebuilt (don't forget the vacuum piston). | 
New well plugs are tapped into place (idle well plugs are outside of the main well plugs). |
The new boosters are wedged in place with a special tool made by The Carb Shop. | 
After the first .005 pass of the mill's surfacing tool, look how uneven the face of the body was were it meets the metering block. The Carb Shop eventually milled .015 from the surface to make it perfectly flat. | 
The old-style brass float (left) could crack along the seam and fill with gas. Holley's new nitrophyl plastic float (right) can't fill up because it is made of a solid material. |
The new needle and seats go back into the bowl, as does each float. | 
Adjustment to the float is made with the needle and seat screws, and this is where The Carb Shop likes to set the float for reassembly. | 
The idle feed restrictors are removed from the metering blocks and moved to a new location that is more sensitive in metering the fuel. |
A common mistake in carb rebuilding is forgetting to put the accelerator pump discharge needle back in! | 
A new power valve (with a 5.5 spring tension) is installed. A smaller power valve is used when the cam has less vacuum (like with a long-duration cam). | 
A two-part epoxy is dabbed onto the boosters to ensure they won't move in the future. |
With a new gasket in place, the base plate goes back on. | 
The choke rod goes back on... | 
...and the choke butterfly goes back on (using threadlock on the screws). |
The bowls go back on with the new gaskets on each side of the metering blocks. The Carb Shop prefers bolts instead of screws to hold the bowls in place. | 
One of the more common mistakes is that most folks forget this tiny gasket on the thermostat housing. | 
Align the choke rod under the choke lever (which is red on this carb) before tightening the three housing screws down. |
Reassemble the thermostat housing (gasket, housing, metal retainer) and put the electrical wire back on. Next, add the ported vacuum source line (which goes to the distributor with vacuum advance). After adding the fuel inlet nuts, the carb is complete and ready to get tested on the dyno. | 
The Carb Shop's O.J. Bretzing adds the Holley 600 to the shop's test mule motor: a 460 big-block Chevy. | 
From the "driver's chair," George Vrbancic makes as many pulls on the water-brake dyno as needed to test the carburetor. |
The first image that comes up on the dyno (The Carb Shop uses DTS Dyno Programs) is the Run Screen, which shows engine rpm and torque sweep gauges on the right, plus engine vitals such as oil pressure, water pressure, and horsepower across the bottom. There are seven pages of information the computer can spit out with each of the next two screens. | 
The Numeric Information chart shows the breakdown of each dyno pull, peak horsepower and torque, plus vitals such as fuel consumption in both pounds per hour and gallons per hour (showing the carb's fuel curve), plus oil pressure and temp, and the water temp of the engine. | 
The last chart is the Bar Graph, where the top line (on left) is the corrected torque numbers, the second one is raw torque numbers, and the bottom two are corrected power (horsepower) and uncorrected power curve. The 600 Holley bolted to this motor produced 541 lb-ft of torque at 4,600 rpm with 473.7 corrected hp, with a peak of 552 corrected hp at 6,000 rpm. But the best info was the fuel pounds-per-hour info, which showed a smooth and consistent transition throughout the rpm range without any spikes or drops in the fuel curve. |