Reducing MP 1" from WOT will not give proper FF change readings as the
fuel enrichment feature causes a larger drop in FF than MP as the
throttle is first retarded. This results in a a leaning effect and the
mixture is not staying the same.  The drop in FF as RPM is retarded is
a much more accurate measure as the the change in the engine driven
fuel pump allows the mixture to remain fairly constant.

Walter,

 

Within the limitations of our on board instrumentation, are you saying that the FF would not be within 10% of each other during this diagnostic (if everything was working as it should)?

 

The last time I looked at my fuel pressure, as measured by a tee off the engine driven pump outlet, it did not change over a drop of 100 RPM and, in any event, remained well above the pressure necessary for the throttle body to correctly meter it to the spider.  Of course, in both cases the Bendix throttle body venturis do see a slight drop in pressure as the airplane slows.

 

I agree the mixture must change as there does seem to be a slight change in the EGTs (as measured by my "instrumentation").

 

More on MAP meters later!