How can a fuel injection engine change to a carburetor?
It depends on the engine and it's more common to go from carburetor to fuel injection not the other direction. On some engines you would still have an ECU for spark and timing control or the transmission. On other engines it's relatively straight forward.The lower intake has to be changed for one with a place to mount a carburetor. On some throttle body fuel injection setups you can swap one for the other without changing the intake. In most cases a different manifold is required.The fuel pressure for fuel injection is higher 40+psi, a low pressure pump is ideal and a fuel pressure regulator for carb level psi around 5psi. Is needed. The fuel rails or fuel lines need changed at least for the last few feet.The ignition system needs to no longer be computer controlled. That might mean an MSD style ignition box or just a coil and distributor swap, it depends on the engine and what it originally had.Your ignition and computer are probably the hardest parts, getting the engine to fire up again might take some extra effort.Air, fuel and spark will all need some attention. Going from carb to throttle body fuel injection is more common lately and it might be less effort to upgrade your fuel injection from stock to something better instead of going to a carb.On a small block Ford or Chevy it's really pretty easy to convert and probably -$1000 especially if you use used parts you could probably convert for under $500.Finding out what your car needs to start without an ECU or how to confuse it to be able to start is something you'll have to look up for the specific engine/car in question.On some engines the conversion is probably impossible without fabricating an intake manifold. Others, you can find the parts on Craigslist or the junkyard and it might only take an afternoon or two.