You might try a 22-12 as that would be less load than a 22-14 but that engine should run fine on a 24-10. Exact engine RPM will depend on how the individual prop loads the engine. If that engine is in good condition and you feel that carb issues have been dealt with, you might try looking at the reeds. A leak there would cause problems. Sorry, I don't want to be overly negative, but my friends Q75 went back to B&B twice and a simple problem was not solved. I don't have a lot of confidence there.

Is the engine cooled well ... that is air directed through the fins and not just around them?? Big round cowl planes can cool well or poorly ... depending on airflow. Scale modelers are famous for not having enough cooling inlets or outlets ... much less flow direction in between. Do you have a scale engine in front of the 75? I lean towards your engine being both overloaded and poorly cooled as the problem. Wish it was here so I could look at it!!!