The Q75 is equivalent to a Sachs 4.2 and we always ran 22-12 on those. I would expect roughly the same rpm with a 24-10 or 22-12 and would
agree that a 22-14 would be a pretty heavy load for the 75. I think 6500 would be a reasonable figure for that engine to turn. As for the performance
drop after a few minutes in the air, that is an odd one. I agree that the reeds would be a good thing to check but they, like the carb, usually work
or they don't so doubt if that's where the problem is. However I learned a long time ago to never say never. You might also check the carb block to
make sure it isn't warped a bit and that the gaskets are good as this sounds more like a heat related issue.