There are three stages of a carb.
The carb body has to be right. If the carb body is wrong then it isn't going to produce the same amount of properly mixed fuel/air into the engine as a "right" body. The body can differ because of the manufacturing tolerances, or the condition of the casting and the surfaces, rough or smooth.
Then the jets have to be right. The jet needs direction, size and taper. Again the differences are very small due to manufacturing tolerances. One effect of a good jet in a good body is that the bit that gives you a correct flow into the airstream (the sweet spot) is larger and easier to find than in a carb where all the bits are just mismatched. It also has to produce a good spray pattern
Lastly the carb needs a good fuel supply. That is, it has to deliver fuel at a constant pressure and a correct pressure and a reliable pressure all the time. Again, this can be affected by manufacturing tolerances and even by changing the parts as they wear.
What you want is a carb that will produce the same result every time you open or close the throttle. You also want a carb that will give you an optimum fuel supply within the limits of the carb. If you read any of the 'real' carb tuning guides (EC Birt for example as recommended earlier) they check all these factors and alter them to produce an optimum. So Birt measures the spring pressure in a Tillotson carb and adjusts the spring so that the pressure falls within tolerance if it is outside it. He examines the pattern of the spray produced at the jet and fits or makes different nozzles/ needles to improve the performance.
In many classes the closest you can get to this is to buy a new bit and hope that it is better than the one you are replacing, because you aren't allowed to modify the carb in any way except by replacement. (Rotax? Blue, TKM etc). One suspects that sometimes these bits do get modified by people with the experience and equipment and they don't get caught. It would require examination with a microscope to find that a body had been remachined to make it run better (ie, machined again over an already machined surface) provided one stayed within the tolerances. Obviously machining bits that shouldn't be machined or machining over tolerance is detectable.
And, if you employ someone to examine your spray pattern and select a different jet/needle from his store of hundreds then you are going to pay for time, knowledge and equipment.
Of course, if a really good carb can make a tenth of a second per lap over a merely average one, then to be of any use to you, you have to be able to produce consistent laps to a tenth. And if a carb produces fluctuating results, you don't really know whether it was your driving or the carb that was at fault.
Even with all the care you can muster, things go wrong. A certain driver (with a national number) went out and was highly placed in one one heat. For the next heat the kart would run intermittently then cut off, requiring a handchoke to keep going. When we took the carb apart we found that the gasket had been inserted upside down and was covering the pulse hole. In theory the result should have been the same for both heats, an intermittent engine. If the carb can operate sometimes with such a big anomaly, can one conclude that all the fine tuning mentioned doesn't really have any effect?
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