a 24x8 is under propped.
The ignition timing should not be a big deal. Maybe 100 rpm max power or so. The weird carb tuning worries me a lot more. The engine behaviour looks like lean. It might be a leak in the T-line connection, or a clogged inner carb filter. Further investigation could include the internal carb lever height (too low), or aerodynamical forces reducing the cowl pressure in flight. These act on the regulating membrane, and can cause the carb to go lean. The prosport has a fitting on the membrane cover. Fit a line to it, and route it to a static pressure tube as used for glider variometers.