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!
