A carburettor (2) to supply a mixture of fuel gas and combustion air to a reciprocating internal combustion engine. The carburettor comprises a venturi (4) and a throttle (10) including a throttle valve (24). A tear-drop shaped obstruction (36) is mounted in the venturi throat (18) on diametrically opposed radial arms (66). The narrowest part of the tubular gap (46) between the walls of venturi (4) and the obstruction (36) is at (48). A continuous slot (50) in the throat wall (18) encircles the obstruction and opens into gap (48). Another continuous slot (70) encircling the obstruction (36) is formed therein and also opens into the gap (48). Fuel gas is supplied to the slots (50) and (70) through passages (52, 74 and 72).