instruments and controls conventional for the operation of the aircraft the instrument panel 10 includes for example a manual throttle control 12 a manual mixture control 14 and a magneto control switch 16 which also controls the economizer unit of the invention as shown in the block diagram of fig2 switch 16 may be coupled to an appropriate pressure switch 100 which in turn operates a switch 102 between first and second positions in the second position labelled 34 lean 34 in fig2 the switch 102 is connected to a 35 u00b0 ignition advance control 104 and to a control 106 which sets the fuel to air ratio at 75 of stoichiometric or 0 05 the other first position labelled 34 both 34 in fig2 switch 102 is connected to a 20 u00b0 ingition advance control 108 and to be manual mixture control 42 this is the normal operation mode presently used the controls 104 and 108 may be coupled to the aircraft magneto so that the magneto is set to establish the ignition at the desired angular advances before top dead center therefore when the pressure switch 100 permits the switch 102 to be in the illustrated position the aircraft is set to the lean fuel to air ratio and to the 35 u00b0 ignition advance so as to achieve a maximum fuel saving during operation on the other hand when the switch arm 102 is set to the other position the magneto is set to the normal 20 u00b0 ignition advance and the manual control 42 is activated to control the fuel to air ratio an appropriate interlock 110 is provided in.