Just for an experiment, quite a few yrs ago, I took the shroud and fan off a 250 Rotax in an Elan to see how it would work as a freeair engine, just for laughs. Thermocouple under the spark plug. Winter around the freezing point. Idled around 300 F Going along with the clutch in ,around 15 MPH. around 350 F. Any more throttle than that, temp would go up like the second hand on a watch. Back off and it would go down just about as fast. I never let it get over 400F. As you say, on the ground tests are useless if you land and take the temp. Going by plug colour is still the best method of setting the carb.