Originally Posted by
ShyTorque
I've been building and tuning up my own 4 stroke engines for many years and have often experimented with carburettor swaps on them. If you fit a carburettor with a smaller Venturi to any engine (which is sometimes done to improve low speed drive ability and response, but will possibly restrict top end power) you need a smaller main jet, otherwise the mixture will be too rich. This is because for the same airflow, the pressure drop across the main jet is greater with a smaller Venturi. Obviously, the basic principle of a carburettor is that the pressure drop across the main jet is what draws out the fuel.
If ice forms in the Venturi of a carb such that it reduces its diameter, the situation is the same as the above.