Looking at how carburetors work: These work off vacuum. The fuel isn't necessarily pulled from the carburetor, it's boiled out of the carburetor. The more vacuum there is, the faster the boiling occurs. The more surface area exposed from the jet, the more fuel can be boiled at once.
Lets go back to the scenario of the different sized engines with the exact same carb configuration.
We'll say the set up is a mikuni vm 34.
YZ125 with 34mm. This engine would use something around a 300-330 main jet
Tecate 250 with a 34mm. would require around a 250-280 main jet
Polaris 400l with a 34mm - comes stock with a 200-210 main. <-
The YZ is going to be the smallest. With the large carb, the amount of vacuum it is pulling is going to be low. It's going to need more surface area on the jet in order to get enough fuel out of the carburetor. The high end RPM needs to breath, so the big carb is an advantage.
The Polaris, a big 400 2 stroke, has a big bore and a carb that is a little bit small for it. It's going to pull a massive vacuum on the carb, boiling much more fuel out of the carb. In turn, a smaller jet is needed.
The Tecate, in the middle, would require a jet between the two.
Restrictive intakes can cause a larger vacuum. It an idle, you actually have the most vacuum, so the main jet is plugged with the needle and the machine operates off the pilot jet. At WOT, which the carb all the way open, your vacuum is going to drop, Tons of air can move it due to the carb slide no longer restricting the air flow.
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