Roy,
It may be a clever girl who enlightens you.
This has puzzled me. I am a Fuel Injection person (Jet engines, IO-540s
and the like).
He had an interesting thought based on his empirical data. The float is
the problem. It is riding too low in the bowl thereby allowing the level
of fuel to be a little too high. When the throttle is closed there is a
momentary lower pressure in the throat of the carb and sucking excess
fuel. Depending on how rich the charge is it may cause it ti stumble.
I was just looking into my Powerplant textbook. There is an Economizer
system that adjusts the mixture based on the throttle setting. Richer
for max power and leaner for cruise. If this is set up wrong it could
stumbler.

Either way the carb is most likely suspect.
Michelle

Roy Page wrote:

We have the same problem with two Archers and a 180.
The problem existed before and after overhaul of both 0-360 engines [1 by
Lycoming & 1 by Signature]
Both rebuilds had overhauled carbs fitted.
The other Archer with 2100 SMOH has the same stumble at or near 1500 RPM.
A clever guy on the net will soon enlighten us as to the cause I am sure ??