I have long toyed with the idea of putting an EFI system on an O series Lycoming/TCM. There are a some commercial systems now available, such as SDSEFI and some DIY solutions such as megasquirt, in addition to the automotive OEM based systems. One thing that always bothered me is that they all need a continuous supply of electricity, so if you have a total electrical failure then your engine dies. Logically speaking, I know that a total electrical failure is pretty rare, and that redundant electrical systems are a possibility, but there is still something appealing about having a mechanical backup. My thought was to build a system that uses an existing carburetor as a throttle body, running it in cut-off when the EFI system is operating. One could add a TPS to the throttle shaft with some simple fab work. It would be heavier than a standard throttle body, but would give you redundancy in the case of any failure with the EFI system - just shut off power to the EFI and push in the mixture knob to start running on the carb. Can anyone see any major drawbacks to this kind of system?