There are combinations that can cause the mixture to be not ideal at different throttle settings

One is tank height. Too high and you will have a rich midrange. Too low and your mid range can be lean.

The ideal tank height is even with the needle valve for an aerobatic plane.

A too large propeller will cause a rich mid range and likewise, too small of a propeller will cause a lean mid range.

The ideal propeller for the Saito 91 is a 14-6.