This is a normally aspirated Lyc, o-360. installed in a certified aircraft. Leaned IAW Manufacturer specs.
No EGT gauge. Due to the rising cost of fuel, leaning has become a "must do" kind of thing.
Had it been an IO-360, it could have been run "lean of peak" which would have done a better job of scavenging the lead, and not caused the fouling which led to the sticky/burned valve.
For those not familiar with "lean of peak" operations. The object of the game, is to lean the mixture untill the EGT peaks, (highest temp) then continue to lean untill it drops, by some amount (between 10 and 20 degrees). Problem is that carburetted engines, can't do that. There will be one cylinder running way rich, and one cylinder running way lean, and the others falling somewhere between, and one of those will be right at peak. With fuel injection, (port, not TB) you can get an even fuel flow to all cyls. Then you can successfully run LOP. Cars do it with a computer, and myriad sensors, piston single aircraft do it with a knob on the insrument panel, and an EGT gauge. Generally, once you reach your cruising altitude, you lean the fuel mixture untill the EGT gauge reaches it's highest temp, then continue to lean, untill the temp falls back down by some number of degrees. As long as all cylinders are running within a few degrees of each other, all is well.
Problem is with a carburetted engine, you cannot get all cylinders running at the same temp, so you must run them all rich. (some richer than others) Generally the ones that present the worst lead fouling problems are the ones running the coolest. Not running hot enough to burn off (scavenge) the lead, and the lead gets deposited in places that it does not need to be, such as valve stems.