Conventional diaphragm carburettors are designed with a butterfly throttle valve on the engine side of the most narrow inlet section (“venturi”). This makes it easier to control fuel flow when the butterfly is closed, but the butterfly and butterfly shaft disturb the air flow in the inlet section. Usually this is compensated by using larger inlet diameters, but in some cases, such as kart racing, this was not possible, because the diameter was limited for regulatory reasons and the use of a butterfly (rather than a slide valve) was mandatory.
Our solution to this was to make a carburettor with a butterfly valve on the inlet side of the venturi in combination with a specific fuel delivery setup that controlled fuel flow when the butterfly was closed. The carburettor was complex, but it was fast and raced by later Formula 1 drivers.