Richard schreef:

Not bein' a pilot (I just jump out of them), doesn't lean cause an EGT
rise? Isn't there some risk of overheating a valve or somesuch if you
take it too lean?

Leaning does cause a rise in EGT - up to a certain point, which is the
point we want because it means optimal combustion, thus minimum fuel
burn for the HP. Leaning any further will make the engine very
inefficient, even to the point where it stops.

On a sidenote, many engines (especially the US-type aircooled boxers)
rely on a bit of excess fuel for valve lubrication so we get recommended
to keep them on the rich side of the EGT peak.