Why are some fuel-injected engines started with the mixture at idle cut-off? The POH for a C172N with a carbureted Lycoming O-320 says:


  STARTING ENGINE
  
  
  Mixture -- RICH
  Carburetor Heat -- COLD
  Master Switch -- ON
  Prime -- AS REQUIRED
  Throttle -- OPEN 1/8 INCH
  Propeller Area -- CLEAR
  Ignition Switch -- START
  


The POH for a C172S with a fuel-injected Lycoming IO-360 says:


  STARTING ENGINE (With Battery)
  
  
  Throttle -- OPEN 1/4 INCH.
  Mixture -- IDLE CUTOFF.
  Propeller Area -- CLEAR.
  Master Switch -- ON.
  
  [Beacon and priming steps omitted]  
  Ignition Switch -- START (release when engine starts).
  Mixture -- ADVANCE smoothly to RICH when engine starts.
  


Why is the fuel-injected engine started at idle cut-off?
Often times fuel injected engines can be difficult to start with the mixture rich, particularly if run recently and still hot.  Following a priming procedure with the electric boost pump then cranking with the mixture at idle cutoff allows the engine to flush out fuel from the injector manifold until an optimal fuel air mixture is reached and the engine starts up.  Quickly moving the mixture to full rich at this point will stabilize the engine at idle with an appropriate throttle setting.