QDM
A carburetted aircraft without EGT gauge is not easy to lean as you have to do it by the ear and the RPM gauge. The rule of thumb is that above 5000’ MSL you should lean to best RPM and then enrich a bit. Your mistake was to go full rich at 6500’ with carb heat on, actually, your carb. was at an higher alt. then the aircraft so the mixture became very rich. Always move engine controls in a slow movement.
Slim S.
One correction to your posting. The engineer can not adjust mixture for any power setting other then idle.