There is provided a method and apparatus for delivering a substantially constant ratio of two gases to an air intake where a varying suction exists. One gas is passed through an inlet port (10) of a body to the inlet end of a passageway in a rotary valve (80) captured within a suitable chamber (73) in the body, thence to the outlet end of the passageway, thence through an outlet portion (69), thence to the air intake. The other gas is passed to a first tube (95) extending along the axis of the rotary valve within the passageway, with a second tube (97) closely enveloping the first. One tube rotates with the rotary valve, the other is fixed. Both tubes have apertures which are in registry when the rotary valve is oriented such that the passageway communicates with both ports. The angular orientation of the rotary valve is adjusted in such a way as to maintain a substantially constant pressure drop between two points respectively upstream and downstream of the rotary valve.