Dude ... the muffler will get VERY hot even if the engine is running rich! I learnt that from my new Saito and I got marks on my forearm to prove it!!! Hehehee ... its the engine head that you want to take the temperature from. What Raideron says might be true, the engine might be leaking somewhere. If there is a leak most times the engine will still idle for a little while if you cover the carb with your finger.

Your definition of hot may not be the same as mine. Try to see how hot the head gets. I always check my engine head temp after I land, I can touch it for like 2-3 secs. That will usually mean only about 200-210 degrees F. For RC cars its easier, we fit a temp sensor to keep the engine at about 220-240 degrees F.