Originally Posted by Captain Smithy
But before power checks or advancing the throttle you must put the mixture back to fully rich, otherwise the engine will start to pink, as you found... you probably didn't damage the engine, I think repeated abuse would cause a failure, but you won't be able to tell until the donk one day suddenly goes bang.
That said there are signs of repeated running on too-lean a mixture, valve seats getting burned etc.
One can do "power check" at 1700 rpm or whatever is prescribed by the engine manufacturer with mixture leaned. It's just a matter of how much is it leaned. If it's too much, you may not get 1700 rpm at all - in this case, just move the lever towards the rich side until you do. I doubt you can ruin the engine by doing a power check with mixture leaned to max RPM/peak EGT - if you can do the same method (best power/best economy) in the cruise at
much higher power settings, why would it hurt on the ground (provided CHTs are observed)?
Burned valve seats are usually signs of consistently
extremely high CHTs (450°F and higher) and you will usually see this temperatures in prolonged climb at Vy or slower (depends on the engine and the engine cowling) or perhaps in poorly cowled engines in cruise with mixture set to best power (50° - 100°F ROP). If the engine is capable of (better said: if air/fuel mixture is consistent in all cylinders) running 50°F lean of peak, you will get very low CHTs than 50°F rich of peak - with the same EGT. Lycoming itself supports leaning techniques and if you read their Flyer publication, you will see that you can set whatever mixture you like (provided the engine isn't running rough) with power settings below 75%, but usually in terms of SFC, leaning too much on the lean side isn't very productive, since power starts to decrease quite rapidly after let's say 50°F LOP.