Mixture and EGT

[Harmon Sommer]

I have heard that in larger piston engine aircraft (larger than the J-3 Cub
I soloed decades ago) that the pilot has a fuel mixture control. Procedure
is for takeoff and climb out to be made with a 'rich' mixture and, after
level off, mixture is leaned out until engine roughness is detected (EGT
increases as mixture is leaned) and then mixture is made richer for ~75
degree decrease in EGT.

My understanding is that combustion temperature is highest at stoich and
declines as mixture is made richer or leaner. Question: why is EGT increasing as
mixture is leaned. Is it because combustion temp is increasing as rich
mixture is brought nearer to stoich? Is it because lean mixture burns
slower and is still burning as the exhaust valve opens. Or none of the
above?

Thx, Harmon