An electrical fuel injection system incorporates electromagnetic injectors wired in two groups and which are operated on alternate cycles during low speed and normal operation. With increasing power demands, the fuel injection computer supplies pulses of increasing length to keep the injectors open longer and supply more fuel to the engine. Two trains of timing pulses are initiated by reed switches operated by the distributor, and each is connected to a pair of latch circuits operative to provide timing pulses on one channel through gate devices to one set of injectors and to block pulses on a second channel to the other set of injectors. The subsequent signal from the other reed switch results in a timing pulse in the second channel. These timing pulses are supplied to the fuel injection computer to initiate the injection pulses and to AND gates which also receive the timing pulses. Under high power demand conditions, an injection pulse may become longer than the time interval between the two trains of timing pulses, and hence a timing pulse is overlapped by an injection pulse from the opposite switch. When this happens, the associated AND gate provides an output pulse to one of two additional latch circuits to cause this latch to provide an output effectively blocking one of said channels and switching additional gate devices to enable both channels to be energized simultaneously by the injection pulses in the second channel. The length of the injection pulse is also measured by any of a plurality of disclosed devices such that when the pulse length drops to approximately 40% of the duty cycle, an output is provided which resets said additional latch circuits to cause said circuits to switch back to two-group operation.