HOT HOT HOT

It was an unusually hot 90+F and I fired up an ST 35C. The engine had run perfectly a few days earlier. It started to lean out and continued to run but at a "much too lean" setting. Not enough power to do anything. The guy flying had little experience and so the plane went round and round for what seemed forever. Probably got a 10 minute run out that tankfull. The engine still had compression after it landed but the head and part-way down the case had changed color to a nice brown shade.(Anyway to fix this???)
The next engine up I set needle valve quite rich on the ground ( ST G21 40) This engine always runs great. Even with the rich setting it leaned out and flew lamely. ( No discoloration on this one).
Third engine, OS 25 SLR I set at very rich and still the engine leaned out.
I was using 10% fuel on all flights.
Is this hot weather the time to increase the castor in the mix? If so, how much? Any other suggestions?

Hi Norm Here in Vancouver we are suffering the very same heat of course and that thinner air requires more revs just to, fly the same lap times. My solution was to shorten up my pipe ,because just going leaner will only get you a leaner run and you can cook your engine. One easy solution can be just upping the nitro,but be ware of thew fact that you will lose fuel economy.Keith V

I noticed the same thing. I started my engine and it sounded a little rich. I pinched off the fuel line to make it lean out then it went back to running rich again. I figured it was fine no need to adjust the needle. The airplane took off, the engine leaned out and it flew like lightning for what seemed like 15 minutes. When it landed Phil said, "do you have a 1/2 gallon fuel tank on that thing." My McCoy 35 Red head is now a McCoy 35 BROWN HEAD.