RE: Engine test stand (Show and tell)
After looking at sound wave charts and seeing those revs per second results it helps to put the task of fine tuning the ignition timing into perspective. Your 25 will run best at about 400 revs per second. The portion of a single cycle where hitting the piston produces power is just after it moves past TDC by some degrees and the striking force should be pretty much absorbed by the piston and sent to the crank by the time the exhaust port opens. So this portion of the cycle, this window of opportunity takes a millionth of a second. Looking at it like this, what are the chances that the factory compression setting and liner height is dead on perfect? A millionth of a second either way from perfect is........less than perfect.
Adjusting prop load [usually by shaving those blades] also tunes the engine in a way by allowing higher timing and more compression to work for you. It is like balancing a balloon on a stick.