Hi, well, with carburettors, they work by causing a depression of air at the venturi that causes a smooth flow of air into the engine, this also draws in fuel from the main jet continiously during the depression, changing the depression by adding foced induction (turbocharging), causes a much greater depression therefore more fuel is needed, so altering the size of the main jet allows you to tune the fuel/air ratio, you can set up the carb by 'jetting' at a rolling road, RMJ.