I have an issue with my AA5 that arose rather suddenly. I have only a single cylinder CHT/EGT gauge that monitors #4 and while flying back from Knoxville to Birmingham, I noticed my exhaust temperature rise sharply higher than usual. Typically, my #4 cylinder runs between 1390 and 1420 degrees in cruise, depending on how rich I run the engine but during a short climb to about 3500 feet, I noticed it was at 1586 and climbing more quickly than the aircraft, reaching 1600 degrees. I have just read that as long as exhaust temperatures don’t go above 1650 than that’s okay. I am concerned because this has not been normal behavior for my plane since purchase last year. It has a young engine – about 125 hours and I confirmed today that the issue still exists. Can it be the extreme cold? And if so, why isn’t CHT affected (still cool – about 360 while EGT fluctuates with engine RPM). When I climb at a constant throttle setting, RPM is reduced and CHT follows accordingly. When I descent (also without changing the throttle) the CHT climbs as the propeller RPM increases. I can get large swings in temperature by manipulating the mixture but never before this great, nor can I get the temp below 1460 in cruise. Since this started, my fuel burn is increased by more than a gallon an hour. Everything else is normal – magnetos, oil temp, oil pressure, fuel pressure, charging and engine operation. There is nothing under the cowling that shows any damage or evidence of something wrong, but something most certainly is. What is happening?