ORIGINAL: w8ye
I lean'em till it starts to slow and then back out the needle some. Works like a charm. You have to be quick on the too lean part on a four stroke or it will quit on you. The Y-S will even throw the prop.
I might do this a couple times to assure myself that I'm where I want to be. You do it for a few years and you will get into the routine.
Been doing it for a long time, but my ears just don't work that way. $20 and a pair of batteries each year removes all of the guess work for me. Why attempt to dissuade someone from getting the tools they need to "calibrate" their ears? You talk about how a person learns to check temps of molten iron by using a gauge, and can then do it my eye. Well, my experience with estimating cloud heights is the same - - I needed a ceilometer to give myself a reference before I could accurately work without one. Two-stokes provide that reference via their sound, but four-strokes don't (for most people).
Like I said, not necessary for two-stokes, but necessary for four-strokes.