Posted for Gary Casey <glcasey@adelphia.net>:

 The LOP discussion centers mostly on fuel injected engines, but  John's
observation with a carbureted engine is a good one.  Left out  of most
discussions is the "rest" of the fuel distribution question.   Yes,
cylinder-to-cylinder variation is usually the big one, but there  is also the
issues of time-dependent (cycle-to-cycle) variations and  in-cylinder
variation.  If you built a (carbureted) plexiglas intake  manifold (I did) you
would see something truly disgusting - droplets  of fuel everywhere, little
rivers running in all directions, puddles  here and there and worse.  I submit
that the "roughness" that is  usually considered the lean limit for carbureted
engines is primarily  from cycle-to-cycle variations in mixture.  On the lean
side of peak  the power produced is directly proportional to the quantity of
fuel.   On the rich side it is mostly independent of the fuel and is
 dependent mostly on the quantity of air delivered. So when the engine  feels
rough it is mostly because at least one cylinder is lean of  peak and the
time-varying fuel delivery is causing a cycle-to-cycle  power variation.  And
some engines are better than others and I  suppose they will operate with all
cylinders lean of peak as reported  by Walter.  There is an unlimited number
of tricks that can be played  with a carburetor installation to improve
cylinder-to-cylinder and  cycle-to-cycle distribution, but one you can try
with any updraft  Lycoming is to vary the throttle opening slightly.  The
airflow won't  be affected, but the angled throttle blade will deflect the
fuel  delivery slightly, changing the engine behavior.  Adding heat always
 helps.
 
 In-cylinder distribution is another variable, but a much more  difficult one
to manipulate.  The biggest variable is the mixture at  the spark plug during
the ignition event and having two ignition  sources helps reduce the
variation.  Incidentally, the best shot at  improving this is to make sure the
fuel is in vapor form during the  intake stroke, increasing the chance it will
be uniformly mixed with  the air.  Injected engines are at a disadvantage here
and the primary  method of evaporating the fuel is to spray it directly on a
hot  surface often by aiming it at the intake valve head and stem.  During
 the dwell between intake strokes the fuel is evaporated.  The worst  time to
inject fuel is during the intake stroke as that fuel has the  least chance of
evaporating.  Our air bleed injectors don't do much  as there is virtually no
air pressure drop to provide the atomization  power.  However, that doesn't
matter much as I have found that a  solid stream is almost as good as a finely
atomized one - it gets the  fuel solidly deposited on the hot surface,
speeding evaporation.
 
 The PRISM system continues to be interesting, but I wonder if the  eventual
market it will find is different than some think.   Turbocharged engines have
limited need as they operated at  essentially a fixed altitude, so
conventional fixed timing and crude  mixture controls maybe aren't so bad.
 Think about the opposite  application - one that might be the "standard" -
build a high- compression large displacement naturally aspirated  engine
(where I  came from "NA" stood for naturally, not normally aspirated) running
 the modest-octane lead-free fuel of the future.  With most operation between
10,000 and 15,000 feet the PRISM system could make a truly  remarkable
difference in engine operation.  And to add a little icing  on the cake an
engine operating under those conditions will emit very  little CO and HC and
even NOX emissions will be less than half those  of a more conventional
engine.  The feds would be happy.
 
FWIW
 Gary Casey