I previously posted:

<<There is a phenomena that
changes the air/fuel ratio, primarily on 4-cylinder engines, in that the air
flow pulsations can get quite large and can possibly go negative and these
pulsations are more pronounced at full throttle.  The result is that full
throttle mixtures are sometimes richer than part throttle mixtures.>>

Scott replied:

<<Certain comments by Walter, Brent and yourself just don't quite match what
I see in my airplane.
 Nobody wants to believe that my fuel flow increased when the rpm was
reduced by 100 or the WOT MAP was reduced 1".  It did though, and my
throttle body was overhauled just 2 years ago.
Air flow not have an affect?  Hmmmmm, you ought to watch my engine
instrumentation as I level out after leaning in climb.  Dramatic change from
the increase in air speed, thus the increase in ram air to both the
induction system and injectors.>>

I probably didn't complete the description:  What I meant to say was that
there is nothing in the fuel system itself that is designed to change
air/fuel ratios with throttle position or air flow - they are (typically, at
least) set for a constant air/fuel ratio across the air flow range for which
they are designed.  That is not to say the air/fuel will stay constant
regardless of operating conditions.  The two variables that can affect a
change in air/fuel ratio are intake pulsations and reference pressure
changes.  The air flow pulsation characteristics can change with either
throttle opening (partial throttle tends to smooth out the pulsations) or
rpm, as the tuning and reverberation characteristics change with rpm.  If
the air flow into the servo changes its "pattern" the reference pressure
could change, upsetting the calibration.  In other words, it could be that
the air swirls in a particular way under one conditions and then detaches
from the wall or otherwise changes its flow pattern at another condition.
This is especially true if the air has to make a sharp bend at the servo
inlet.  So what you are seeing could very well happen - the effect just
isn't built into the system.  As an example, my Cardinal has a very nicely
designed servo inlet and the air/fuel ratio is very constant over a wide
range of rpms and throttle openings.  Come help me finish my ES and I'll
report on that...

Oh, and  one more thing - the reference pressure applied to the injectors
does not affect the fuel flow, but it does affect the reading of a
pressure-type fuel flow indicator.  The indicator is a pressure gage that is
referenced to the pressure inside the cabin.  The pressure drop in the
injector itself is, however, referenced to the vent pressure at the
injector.  It is a small effect, but could change your fuel flow readings
slightly.



Gary Casey