My plane has 0470M engines with the Bendix PSD-5C pressure carbs. I only have a single thermistor probe cylinder 3 each engine. While both engine indications of EGT and CHT have matched after start climb cruise the left engine CHT temperature still matches until I lean in cruise. At that point the CHT drops to about 225 . If I push its mixture control to full rich it then mirrors the right . The thermistor probe checks good and gauge has been overhauled. Any ideas what could be causing this change in cruise?