Engine stumble, Any thoughts?

My 172M doesn't have any stumble for some reason. It is the rpm range
generally less than 1500 rpm though, where the carb has to evolve from
the idle jet system to the main jet. Carbs have a problem in this area
because this transistion simply involves a gradual change of the jet
system design, yet has to keep the mixture within a burnable range
(about a 9:1 to 18:1 air/fuel ratio) or there will be a stumble. I
don't know how carb designers (in the 30s I suppose) got it to work as
well as they do.

The main jet system is simple. The idle jet system is always a wierd
collection of ports etc.

Is it a lean stumble? or is it maybe a rich stumble?