Hey TC,
O-320 AIA in my Apache... They stumble when you retard the throttle on
downwind if they are leaned out at all...
The right engine was the worst, it would strangle and shake like a wet
puppy for about 1 second to 2 seconds ... I finally discovered that
carb was the wrong part number for the engine (It was a Cessna carb for
gawds sake) and put the correct carb on which greatly improved the
situation - a thousand dollars later...
Now, if you go full rich mixture for a second, before retarding the
throttle, they do not stumble... If you are leaned out and reduce
throttle at a normal rate they will burble for a second... If you have
been well trained, like my airplane has managed to make me, you
instinctively know to just how fast you can creep the throttle back
through that magic point so there is no stumble when leaned...

My own pet theory, based on a life time of building - and breaking -
engines is that there is a point where reducing angle of the throttle
plate (butterfly), too rapidly and therefore reducing the air velocity
rapidly, where the existing swirl pattern within the intake manifold
collapses, allowing the fuel mixture to collapse before the new lower
velocity swirl pattern establishes itself... It is a lean stumble in
my book..

denny