I have seen this mod on Yamaha XT660 bikes. as I understand it The air temperature has a resistance variable with temperature, on a colder day the air is more dense so the fuel map is changed to make it richer. The idea is that by making the EFI think it is colder than it is, it will make the mixture richer. It improves the running of the Yamahas when a louder pipe is fitted.
I dont have EFI but if I did I would measure the resistance of the sensor in ambient and then stick it in the freezer to see what the value was then, put a resistor of this value in place of the sensor.
