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REFLECTOR: standby vacuum system

  My ignorance here,(somebody teach me), not having flown IFR on my own, or
suffered any failures other than a slow electric gyro failure on a clear
nightflight over flat land, I have two question areas about the Precise
Flight standby vacuum system;

1) Doesn't bleeding air into the intake manifold downstream of the
carburetor screw up the mixture to at least some of the cylinders? lean them
out unevenly?
(applies to throttle body injectors as well, but not to direct port injection)

2) If it really doesn't, then why can't a bleed air (manifold suction/pressure
   differential) system be used all the time, with the normal suction regulator 
   placed handily in line?  

If the Precise flight system gives inadequate vacuum to drive the instruments
when the engine is throttled back,(or is that wide open? I've never had a
manifold pressure gauge to watch) as in a descent, isn't that a failure of your
standby system as well?  If this isn't a problem, why can't it be a primary
system? I won't be flying if my engine isn't turning. (well, not for long.)