one down, two to go...

That is because of the difference in ignition method: Engines running on glow, ignition "timing" is determined by fuel composition, CR, plug heat value and mixture strength.
Of those four variables, only the mixture strength is "on the fly" adjustable, the other three are engine parameters.

So if you lean the mixture on a glow engine, ignition timing changes "immediately", and THAT is what you hear: the "sound" of the engine changes pretty clearly audible.
Gasoline and spark ignition, the mixture strength does not affect the timing. so the change in how the engine runs is not immediately noticable, but with the fuel, also the oil supply changes, and with the reduction in excess fuel that oil also changes in "quality" (less soot and fuel dilution). So the engines exhaust note changes much more subtle, and the change takes more time.

Setting a gasser requires a bit more patience and attention.